Semiconductor device, display device including the semiconductor device, display module including the display device, and electronic appliance including the semiconductor device, the display device, and the display module

ABSTRACT

A change in electrical characteristics is inhibited and reliability is improved in a semiconductor device using a transistor including an oxide semiconductor. One embodiment of a semiconductor device including a transistor includes a gate electrode, first and second insulating films over the gate electrode, an oxide semiconductor film over the second insulating film, and source and drain electrodes electrically connected to the oxide semiconductor film. A third insulating film is provided over the transistor and a fourth insulating film is provided over the third insulating film. The third insulating film includes oxygen. The fourth insulating film includes nitrogen. The amount of oxygen released from the third insulating film is 1×10 19 /cm 3  or more by thermal desorption spectroscopy, which is estimated as oxygen molecules. The amount of oxygen molecules released from the fourth insulating film is less than 1×10 19 /cm 3 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/632,381, filed Feb. 26, 2015, now allowed, which claims the benefitof a foreign priority application filed in Japan as Serial No.2014-039151 on Feb. 28, 2014, both of which are incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice including an oxide semiconductor and a display device includingthe semiconductor device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, and a manufacturing method. In addition, the presentinvention relates to a process, a machine, manufacture, and acomposition of matter. In particular, the present invention relates to asemiconductor device, a display device, a light-emitting device, a powerstorage device, a storage device, a driving method thereof, and amanufacturing method thereof.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A semiconductor element such as a transistor, asemiconductor circuit, an arithmetic device, and a memory device areeach an embodiment of a semiconductor device. An imaging device, adisplay device, a liquid crystal display device, a light-emittingdevice, an electro-optical device, a power generation device (includinga thin film solar cell, an organic thin film solar cell, and the like),and an electronic device may each include a semiconductor device.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor usinga semiconductor thin film formed over a substrate having an insulatingsurface (also referred to as a field-effect transistor (FET) or a thinfilm transistor (TFT)). Such transistors are applied to a wide range ofelectronic devices such as an integrated circuit (IC) and an imagedisplay device (display device). A semiconductor material typified bysilicon is widely known as a material for a semiconductor thin film thatcan be used for a transistor. As another material, an oxidesemiconductor has been attracting attention (e.g., Patent Document 1).

Furthermore, for example, Patent document 2 discloses a semiconductordevice in which, to reduce oxygen vacancy in an oxide semiconductorlayer, an insulating film which releases oxygen by heating is used as abase insulating layer of the oxide semiconductor layer where a channelis formed.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165529-   [Patent Document 2] Japanese Published Patent Application No.    2012-009836

SUMMARY OF THE INVENTION

In the case where a transistor is manufactured using an oxidesemiconductor film for a channel region, oxygen vacancy formed in theoxide semiconductor film adversely affects the transistorcharacteristics; therefore, the oxygen vacancy causes a problem. Forexample, oxygen vacancy formed in the oxide semiconductor film is bondedwith hydrogen to serve as a carrier supply source. The carrier supplysource generated in the oxide semiconductor film causes a change in theelectrical characteristics, typically, shift in the threshold voltage,of the transistor including the oxide semiconductor film. Furthermore,there is a problem in that electrical characteristics fluctuate amongthe transistors. Therefore, it is preferable that the amount of oxygenvacancy in the channel region of the oxide semiconductor film be assmall as possible.

In view of the above problem, an object of one embodiment of the presentinvention is to inhibit a change in electrical characteristics and toimprove reliability in a semiconductor device using a transistorincluding an oxide semiconductor. Another object of one embodiment ofthe present invention is to provide a semiconductor device with lowpower consumption. Another object of one embodiment of the presentinvention is to provide a novel semiconductor device. Another object ofone embodiment of the present invention is to provide a novel displaydevice.

Note that the description of the above object does not disturb theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Objects other than theabove objects will be apparent from and can be derived from thedescription of the specification and the like.

One embodiment of the present invention is a semiconductor deviceincluding a transistor which includes a gate electrode, a firstinsulating film over the gate electrode, a second insulating film overthe first insulating film, an oxide semiconductor film over the secondinsulating film, a source electrode electrically connected to the oxidesemiconductor film, and a drain electrode electrically connected to theoxide semiconductor film. A third insulating film is provided over thetransistor, and a fourth insulating film is provided over the thirdinsulating film. The third insulating film includes oxygen. The fourthinsulating film includes nitrogen. The amount of oxygen moleculesreleased from the third insulating film is greater than or equal to1×10¹⁹/cm³ when measured by thermal desorption spectroscopy. The amountof oxygen molecules released from the fourth insulating film is lessthan 1×10¹⁹/cm³ when measured by the thermal desorption spectroscopy.

Another embodiment of the present invention is a semiconductor deviceincluding a transistor which includes a gate electrode, a firstinsulating film over the gate electrode, a second insulating film overthe first insulating film, an oxide semiconductor film over the secondinsulating film, a source electrode electrically connected to the oxidesemiconductor film, and a drain electrode electrically connected to theoxide semiconductor film. A third insulating film is provided over thetransistor, a fifth insulating film is provided over the thirdinsulating film, and a fourth insulating film is provided over the fifthinsulating film. The third insulating film includes oxygen. The fourthinsulating film includes nitrogen. The fifth insulating film includesmetal. The fifth insulating film includes at least one of oxygen andnitrogen. The amount of oxygen molecules released from the thirdinsulating film is greater than or equal to 1×10¹⁹/cm³ when measured bythermal desorption spectroscopy. The amount of oxygen molecules releasedfrom the fourth insulating film is less than 1×10¹⁹/cm³ when measured bythe thermal desorption spectroscopy.

Another embodiment of the present invention is a semiconductor deviceincluding a transistor which includes a gate electrode, a firstinsulating film over the gate electrode, a second insulating film overthe first insulating film, an oxide semiconductor film over the secondinsulating film, a third insulating film over the oxide semiconductorfilm, a source electrode electrically connected to the oxidesemiconductor film, and a drain electrode electrically connected to theoxide semiconductor film. A fourth insulating film is provided over thetransistor. The third insulating film includes oxygen. The fourthinsulating film includes nitrogen. The amount of oxygen moleculesreleased from the third insulating film is greater than or equal to1×10¹⁹/cm³ when measured by thermal desorption spectroscopy. The amountof oxygen molecules released from the fourth insulating film is lessthan 1×10¹⁹/cm³ when measured by the thermal desorption spectroscopy.

Another embodiment of the present invention is a semiconductor deviceincluding a transistor which includes a gate electrode, a firstinsulating film over the gate electrode, a second insulating film overthe first insulating film, an oxide semiconductor film over the secondinsulating film, a third insulating film over the oxide semiconductorfilm, a fifth insulating film over the third insulating film, a sourceelectrode electrically connected to the oxide semiconductor film, and adrain electrode electrically connected to the oxide semiconductor film.A fourth insulating film is provided over the transistor. The thirdinsulating film includes oxygen. The fourth insulating film includesnitrogen. The fifth insulating film includes a metal element. The fifthinsulating film includes at least one of oxygen and nitrogen. The amountof oxygen molecules released from the third insulating film is greaterthan or equal to 1×10¹⁹/cm³ when measured by thermal desorptionspectroscopy. The amount of oxygen molecules released from the fourthinsulating film is less than 1×10¹⁹/cm³ when measured by the thermaldesorption spectroscopy.

In any of the above structures, the third insulating film preferablyincludes oxygen, nitrogen, and silicon. In any of the above structures,the fourth insulating film preferably includes nitrogen and silicon.

In any of the above structures, the metal element included in the fifthinsulating film preferably includes at least one of indium, zinc,titanium, aluminum, tungsten, tantalum, and molybdenum.

In any of the above structures, the first insulating film preferablyincludes nitrogen and silicon.

In any of the above structures, the oxide semiconductor film preferablyincludes O, In, Zn, and M (M is Ti, Ga, Y, Zr, La, Ce, Nd, or Hf). Inany of the above structures, it is preferable that the oxidesemiconductor film include a crystal part, the crystal part include aportion, and a c-axis of the portion be a parallel to a normal vector ofa surface where the oxide semiconductor film is formed.

Another embodiment of the present invention is a display deviceincluding the semiconductor device according to any one of the abovestructures, and a display element. Another embodiment of the presentinvention is a display module including the display device and a touchsensor. Another embodiment of the present invention is an electronicappliance including the semiconductor device according to any one of theabove structures, the display device, or the display module; and anoperation key or a battery.

According to one object of one embodiment of the present invention, achange in electrical characteristics can be inhibited and reliabilitycan be improved in a semiconductor device using a transistor includingan oxide semiconductor. Alternatively, according to one embodiment ofthe present invention, a semiconductor device with low power consumptioncan be provided. According to one embodiment of the present invention, anovel semiconductor device can be provided. According to one embodimentof the present invention, a novel display device can be provided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views illustrating oneembodiment of a semiconductor device.

FIGS. 2A to 2D are cross-sectional views each illustrating oneembodiment of a semiconductor device.

FIGS. 3A to 3C are a top view and cross-sectional views illustrating oneembodiment of a semiconductor device.

FIGS. 4A to 4D are cross-sectional views each illustrating oneembodiment of a semiconductor device.

FIGS. 5A to 5C are a top view and cross-sectional views illustrating oneembodiment of a semiconductor device.

FIGS. 6A to 6D are cross-sectional views each illustrating oneembodiment of a semiconductor device.

FIGS. 7A to 7C are a top view and cross-sectional views illustrating oneembodiment of a semiconductor device.

FIGS. 8A to 8D are cross-sectional views each illustrating oneembodiment of a semiconductor device.

FIGS. 9A and 9B are band diagrams.

FIGS. 10A to 10D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 11A to 11D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 12A to 12D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 13A and 13B are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 14A to 14D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 15A to 15D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 16A to 16D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 17A to 17D are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 18A to 18C are cross-sectional views illustrating an example of amanufacturing process of a semiconductor device.

FIGS. 19A to 19D are Cs-corrected high-resolution TEM images of a crosssection of a CAAC-OS and a cross-sectional schematic view of a CAAC-OS.

FIGS. 20A to 20D are Cs-corrected high-resolution TEM images of a planeof a CAAC-OS.

FIGS. 21A to 21C show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD.

FIG. 22 shows a movement path of oxygen in an In—Ga—Zn oxide.

FIG. 23 illustrates a calculation model.

FIGS. 24A and 24B illustrate an initial state and a final state,respectively.

FIG. 25 shows an activation barrier.

FIGS. 26A and 26B illustrate an initial state and a final state,respectively.

FIG. 27 shows an activation barrier.

FIG. 28 shows the transition levels of V_(O)H.

FIG. 29 is a top view illustrating one embodiment of a display device.

FIG. 30 is a cross-sectional view illustrating one embodiment of adisplay device.

FIG. 31 is a cross-sectional view illustrating one embodiment of adisplay device.

FIGS. 32A to 32C are a block diagram and circuit diagrams illustrating adisplay device.

FIG. 33 illustrates a display module.

FIGS. 34A to 34H illustrate electronic appliances.

FIG. 35 shows TDS measurement results.

FIGS. 36A and 36B show SIMS measurement results.

FIGS. 37A and 37B each show the electric characteristics of transistorsin an example.

FIGS. 38A and 38B each show the electric characteristics of transistorsin an example.

FIG. 39 shows results of reliability tests performed on transistors inan example.

FIG. 40A schematically illustrates a CAAC-OS deposition model, and FIGS.40B and 40C are cross-sectional views of pellets and a CAAC-OS.

FIG. 41 schematically illustrates a deposition model of an nc-OS and apellet.

FIG. 42 illustrates a pellet.

FIG. 43 illustrates force applied to a pellet on a formation surface.

FIGS. 44A and 44B illustrate transfer of pellets on formation surfaces.

FIGS. 45A and 45B illustrate an InGaZnO₄ crystal.

FIGS. 46A and 46B show a structure and the like of InGaZnO₄ beforecollision of an atom.

FIGS. 47A and 47B show a structure and the like of InGaZnO₄ aftercollision of an atom.

FIGS. 48A and 48B show trajectories of atoms after collision of atoms.

FIGS. 49A and 49B are cross-sectional HAADF-STEM images of a CAAC-OS anda target.

FIGS. 50A and 50B show electron diffraction patterns of a CAAC-OS.

FIG. 51 shows a change in crystal part of an In—Ga—Zn oxide induced byelectron irradiation.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments will be described with reference to drawings.However, the embodiments can be implemented with various modes. It willbe readily appreciated by those skilled in the art that modes anddetails can be changed in various ways without departing from the spiritand scope of the present invention. Thus, the present invention shouldnot be interpreted as being limited to the following description of theembodiments.

In the drawings, the size, the layer thickness, and the region isexaggerated for clarity in some cases. Therefore, embodiments of thepresent invention are not limited to such a scale. Note that thedrawings are schematic views showing ideal examples, and embodiments ofthe present invention are not limited to shapes or values shown in thedrawings.

Note that in this specification, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not limit the components numerically.

Note that in this specification, terms for describing arrangement, suchas “over” “above”, “under”, and “below”, are used for convenience indescribing a positional relation between components with reference todrawings. Furthermore, the positional relation between components ischanged as appropriate in accordance with a direction in which eachcomponent is described. Thus, there is no limitation on terms used inthis specification, and description can be made appropriately dependingon the situation.

In this specification and the like, a transistor is an element having atleast three terminals of a gate, a drain, and a source. In addition, thetransistor has a channel region between a drain (a drain terminal, adrain region, or a drain electrode) and a source (a source terminal, asource region, or a source electrode), and current can flow through thedrain region, the channel region, and the source region. Note that inthis specification and the like, a channel region refers to a regionthrough which current mainly flows.

Further, functions of a source and a drain might be switched whentransistors having different polarities are employed or a direction ofcurrent flow is changed in circuit operation, for example. Therefore,the terms “source” and “drain” can be switched in this specification andthe like.

Note that in this specification and the like, the expression“electrically connected” includes the case where components areconnected through an “object having any electric function”. There is noparticular limitation on an □object having any electric function□ aslong as electric signals can be transmitted and received betweencomponents that are connected through the object. Examples of an “objecthaving any electric function” are a switching element such as atransistor, a resistor, an inductor, a capacitor, and elements with avariety of functions as well as an electrode and a wiring.

Note that in this specification and the like, a “silicon oxynitridefilm” refers to a film that includes oxygen at a higher proportion thannitrogen, and a “silicon nitride oxide film” refers to a film thatincludes nitrogen at a higher proportion than oxygen.

In this specification and the like, the term “parallel” indicates thatthe angle formed between two straight lines is greater than or equal to−10° and less than or equal to 10°, and accordingly also includes thecase where the angle is greater than or equal to −5° and less than orequal to 5°. A term “substantially parallel” indicates that the angleformed between two straight lines is greater than or equal to −30° andless than or equal to 30°. In addition, a term “perpendicular” indicatesthat the angle formed between two straight lines is greater than orequal to 80° and less than or equal to 100°, and accordingly includesthe case where the angle is greater than or equal to 85° and less thanor equal to 95°. A term “substantially perpendicular” indicates that theangle formed between two straight lines is greater than or equal to 60°and less than or equal to 120°.

Embodiment 1

In this embodiment, a semiconductor device of one embodiment of thepresent invention is described with reference to FIGS. 1A to 1C, FIGS.2A to 2D, FIGS. 3A to 3C, FIGS. 4A to 4D, FIGS. 5A to 5C, FIGS. 6A to6D, FIGS. 7A to 7C, FIGS. 8A to 8D, FIGS. 9A and 9B, FIGS. 10A to 10D,FIGS. 11A to 11D, FIGS. 12A to 12D, FIGS. 13A and 13B, FIGS. 14A to 14D,FIGS. 15A to 15D, FIGS. 16A to 16D, FIGS. 17A to 17D, and FIGS. 18A to18C.

<Structural Example 1 of Semiconductor Device>

FIG. 1A is a top view of a transistor 100 that is a semiconductor deviceof one embodiment of the present invention. FIG. 1B is a cross-sectionalview taken along a dashed dotted line X1-X2 in FIG. 1A, and FIG. 1C is across-sectional view taken along a dashed dotted line Y1-Y2 in FIG. 1A.Note that in FIG. 1A, some components of the transistor 100 (e.g., aninsulating film serving as a gate insulating film) are not illustratedto avoid complexity. Furthermore, the direction of the dashed dottedline X1-X1 may be called a channel length direction, and the directionof the dashed dotted line Y1-Y2 may be called a channel width direction.As in FIG. 1A, some components are not illustrated in some cases in topviews of transistors described below.

The transistor 100 includes a conductive film 104 functioning as a gateelectrode over a substrate 102, an insulating film 106 (also referred toas a first insulating film) over the substrate 102 and the conductivefilm 104, an insulating film 107 (also referred to as a secondinsulating film) over the insulating film 106, an oxide semiconductorfilm 108 over the insulating film 107, and conductive films 112 a and112 b functioning as source and drain electrodes electrically connectedto the oxide semiconductor film 108. Over the transistor 100,specifically, over the conductive films 112 a and 112 b and the oxidesemiconductor film 108, insulating films 114 and 116 (also referred toas third insulating films) and an insulating film 118 (also referred toas a fourth insulating film) are provided. The insulating films 114,116, and 118 function as protective insulating films for the transistor100.

Note that in the transistor 100, the conductive film 112 a has atwo-layer structure formed of a conductive film 110 a and a conductivefilm 111 a. In addition, the conductive film 112 b has a two-layerstructure formed of a conductive film 110 b and a conductive film 111 b.Note that the structures of the conductive films 112 a and 112 b are notlimited thereto, and the conductive films 112 a and 112 b each may havea single-layer structure or a stacked-layer structure including three ormore layers.

The insulating film 106 and the insulating film 107 each serve as a gateinsulating film of the transistor 100.

When oxygen vacancy is formed in the oxide semiconductor film 108included in the transistor 100, electrons serving as carriers aregenerated; as a result, the transistor 100 tends to be normally-on.Therefore, for stable transistor characteristics, it is important toreduce oxygen vacancy in the oxide semiconductor film 108. In thestructure of the transistor of one embodiment of the present invention,excess oxygen is introduced into an insulating film over the oxidesemiconductor film 108, here, the insulating film 114 over the oxidesemiconductor film 108, whereby oxygen is moved from the insulating film114 to the oxide semiconductor film 108 to fill oxygen vacancy in theoxide semiconductor film 108. Alternatively, excess oxygen is introducedinto the insulating film 116 over the oxide semiconductor film 108,whereby oxygen is moved from the insulating film 116 to the oxidesemiconductor film 108 through the insulating film 114 to fill oxygenvacancy in the oxide semiconductor film 108. Alternatively, excessoxygen is introduced into the insulating films 114 and 116 over theoxide semiconductor film 108, whereby oxygen is moved from both theinsulating films 114 and 116 to the oxide semiconductor film 108 to filloxygen vacancy in the oxide semiconductor film 108.

Therefore, the insulating films 114 and 116 include oxygen.Specifically, the insulating films 114 and 116 include oxygen that iseasily moved to the oxide semiconductor film 108 in the insulating films114 and 116. Examples of the oxygen include O and O₂. It is preferablethat the insulating films 114 and 116 each include a region (oxygenexcess region) including oxygen in excess of that in the stoichiometriccomposition. In other words, the insulating films 114 and 116 are eachan insulating film capable of releasing oxygen. Note that the oxygenexcess region is formed in each of the insulating films 114 and 116 insuch a manner that oxygen is introduced into the insulating films 114and 116 after the deposition, for example. As a method for introducingoxygen, an ion implantation method, an ion doping method, a plasmaimmersion ion implantation method, plasma treatment, or the like may beemployed.

The amount of oxygen molecules released from each of the insulatingfilms 114 and 116 is greater than or equal to 1×10¹⁹/cm³ when measuredby thermal desorption spectroscopy (TDS). Oxygen can exist betweenlattices uniformly or substantially uniformly in the insulating films114 and 116. Oxygen in the insulating films 114 and 116 is released tothe oxide semiconductor film 108 by heat treatment.

The amount of oxygen molecules released from the insulating film 118 isless than 1×10¹⁹/cm³ when measured by TDS.

Providing the insulating films 114 and 116 over the oxide semiconductorfilm 108 makes it possible to move oxygen in the insulating films 114and 116 to the oxide semiconductor film 108, so that oxygen vacancyformed in the oxide semiconductor film 108 can be filled. Furthermore,the insulating film 118, which releases a small amount of oxygen,provided over the insulating films 114 and 116 can inhibit oxygen in theinsulating films 114 and 116 from diffusing to the outside. The oxygenvacancy in the oxide semiconductor film 108 is filled, whereby a highlyreliable semiconductor device can be provided.

Note that the insulating film 114 can be formed using an oxideinsulating film having a low density of states due to nitrogen oxidebetween the energy of the valence band maximum (E_(v_os)) and the energyof the conduction band minimum (E_(c_os)) of the oxide semiconductorfilm. A silicon oxynitride film that releases less nitrogen oxide, analuminum oxynitride film that releases less nitrogen oxide, and the likecan be used as the oxide insulating film in which the density of statesdue to nitrogen oxide is low between E_(v_os) and E_(c_os).

Note that a silicon oxynitride film that releases less nitrogen oxide isa film of which the amount of released ammonia is larger than the amountof released nitrogen oxide in thermal desorption spectroscopy analysis;the amount of released ammonia is typically greater than or equal to1×10¹⁸/cm³ and less than or equal to 5×10¹⁹/cm³. Note that the amount ofreleased ammonia is the amount of ammonia released by heat treatmentwith which the surface temperature of a film becomes higher than orequal to 50° C. and lower than or equal to 650° C., preferably higherthan or equal to 50° C. and lower than or equal to 550° C.

Nitrogen oxide (NO_(x); x is greater than or equal to 0 and less than orequal to 2, preferably greater than or equal to 1 and less than or equalto 2), typically NO₂ or NO, forms levels in the insulating film 114, forexample. The level is positioned in the energy gap of the oxidesemiconductor film 108. Therefore, when nitrogen oxide is diffused tothe interface between the insulating film 114 and the oxidesemiconductor film 108, an electron is trapped by the level on theinsulating film 114 side. As a result, the trapped electron remains inthe vicinity of the interface between the insulating film 114 and theoxide semiconductor film 108; thus, the threshold voltage of thetransistor is shifted in the positive direction.

Nitrogen oxide reacts with ammonia and oxygen in heat treatment. Sincenitrogen oxide included in the insulating film 114 reacts with ammoniaincluded in the insulating film 116 in heat treatment, nitrogen oxideincluded in the insulating film 114 is reduced. Therefore, an electronis hardly trapped at the interface between the insulating film 114 andthe oxide semiconductor film 108.

By using, for the insulating film 114, the oxide insulating film havinga low density of states due to nitrogen oxide between E_(v_os) andE_(c_os), the shift in the threshold voltage of the transistor can bereduced, which leads to a smaller change in the electricalcharacteristics of the transistor.

Note that in an ESR spectrum at 100 K or lower of the insulating film114, by heat treatment of a manufacturing process of the transistor,typically heat treatment at a temperature higher than or equal to 300°C. and lower than the strain point of the substrate, a first signal thatappears at a g-factor of greater than or equal to 2.037 and less than orequal to 2.039, a second signal that appears at a g-factor of greaterthan or equal to 2.001 and less than or equal to 2.003, and a thirdsignal that appears at a g-factor of greater than or equal to 1.964 andless than or equal to 1.966 are observed. The split width of the firstand second signals and the split width of the second and third signalsthat are obtained by ESR measurement using an X-band are eachapproximately 5 mT. The sum of the spin densities of the first signalthat appears at a g-factor of greater than or equal to 2.037 and lessthan or equal to 2.039, the second signal that appears at a g-factor ofgreater than or equal to 2.001 and less than or equal to 2.003, and thethird signal that appears at a g-factor of greater than or equal to1.964 and less than or equal to 1.966 is lower than 1×10¹⁸ spins/cm³,typically higher than or equal to 1×10¹⁷ spins/cm³ and lower than 1×10¹⁸spins/cm³.

In the ESR spectrum at 100 K or lower, the first signal that appears ata g-factor of greater than or equal to 2.037 and less than or equal to2.039, the second signal that appears at a g-factor of greater than orequal to 2.001 and less than or equal to 2.003, and the third signalthat appears at a g-factor of greater than or equal to 1.964 and lessthan or equal to 1.966 correspond to signals attributed to nitrogenoxide (NO_(x); x is greater than or equal to 0 and smaller than or equalto 2, preferably greater than or equal to 1 and less than or equal to2). Typical examples of nitrogen oxide include nitrogen monoxide andnitrogen dioxide. In other words, the lower the total spin density ofthe first signal that appears at a g-factor of greater than or equal to2.037 and less than or equal to 2.039, the second signal that appears ata g-factor of greater than or equal to 2.001 and less than or equal to2.003, and the third signal that appears at a g-factor of greater thanor equal to 1.964 and less than or equal to 1.966 is, the lower thecontent of nitrogen oxide in the oxide insulating film is.

The nitrogen concentration of the oxide insulating film having a lowdensity of states due to nitrogen oxide between E_(v_os) and E_(c_os)measured by secondary mass spectrometry (SIMS) is lower than or equal to6×10²⁰ atoms/cm³.

The oxide insulating film in which the density of states due to nitrogenoxide is low between E_(v_os) and E_(c_os) is formed by a PECVD methodat a substrate temperature of higher than or equal to 220° C., higherthan or equal to 280° C., or higher than or equal to 350° C. usingsilane and nitrogen oxide, whereby a dense and hard film can be formed.

Other constituent elements of the semiconductor device of thisembodiment are described below in detail.

<Substrate>

There is no particular limitation on the property of a material and thelike of the substrate 102 as long as the material has heat resistanceenough to withstand at least heat treatment to be performed later. Forexample, a glass substrate, a ceramic substrate, a quartz substrate, asapphire substrate, or the like may be used as the substrate 102.Alternatively, a single crystal semiconductor substrate or apolycrystalline semiconductor substrate made of silicon, siliconcarbide, or the like, a compound semiconductor substrate made of silicongermanium or the like, an SOI substrate, or the like may be used as thesubstrate 102. Still alternatively, any of these substrates providedwith a semiconductor element may be used as the substrate 102. In thecase where a glass substrate is used as the substrate 102, a glasssubstrate having any of the following sizes can be used: the 6thgeneration (1500 mm×1850 mm), the 7th generation (1870 mm×2200 mm), the8th generation (2200 mm×2400 mm), the 9th generation (2400 mm×2800 mm),and the 10th generation (2950 mm×3400 mm). Thus, a large-sized displaydevice can be manufactured.

Alternatively, a flexible substrate may be used as the substrate 102,and the transistor 100 may be provided directly on the flexiblesubstrate. Alternatively, a separation layer may be provided between thesubstrate 102 and the transistor 100. The separation layer can be usedwhen part or the whole of a semiconductor device formed over theseparation layer is separated from the substrate 102 and transferredonto another substrate. In such a case, the transistor 100 can betransferred to a substrate having low heat resistance or a flexiblesubstrate as well.

<Conductive Film>

The conductive film 104 functioning as a gate electrode and theconductive films 112 a and 112 b functioning as a source electrode and adrain electrode can each be formed using a metal element selected fromchromium (Cr), copper (Cu), aluminum (Al), gold (Au), silver (Ag), zinc(Zn), molybdenum (Mo), tantalum (Ta), titanium (Ti), tungsten (W),manganese (Mn), nickel (Ni), iron (Fe), and cobalt (Co); an alloyincluding any of these metal element as its component; an alloyincluding a combination of any of these elements; or the like.

Furthermore, the conductive films 104, 112 a, and 112 b may have asingle-layer structure or a stacked-layer structure of two or morelayers. For example, a single-layer structure of an aluminum filmincluding silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a two-layer structure in which a titaniumfilm is stacked over a titanium nitride film, a two-layer structure inwhich a tungsten film is stacked over a titanium nitride film, atwo-layer structure in which a tungsten film is stacked over a tantalumnitride film or a tungsten nitride film, a three-layer structure inwhich a titanium film, an aluminum film, and a titanium film are stackedin this order, and the like can be given. Alternatively, an alloy filmor a nitride film in which aluminum and one or more elements selectedfrom titanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium are combined may be used.

The conductive films 104, 112 a, and 112 b can be formed using alight-transmitting conductive material such as indium tin oxide, indiumoxide including tungsten oxide, indium zinc oxide including tungstenoxide, indium oxide including titanium oxide, indium tin oxide includingtitanium oxide, indium zinc oxide, or indium tin oxide to which siliconoxide is added.

A Cu—X alloy film (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) may be usedfor the conductive films 104, 112 a, and 112 b. Use of a Cu—X alloy filmenables the manufacturing cost to be reduced because wet etching processcan be used in the processing.

<Gate Insulating Film>

As each of the insulating films 106 and 107 functioning as a gateinsulating film of the transistor 100, an insulating layer including atleast one of the following films formed by a plasma enhanced chemicalvapor deposition (PECVD) method, a sputtering method, or the like can beused: a silicon oxide film, a silicon oxynitride film, a silicon nitrideoxide film, a silicon nitride film, an aluminum oxide film, a hafniumoxide film, an yttrium oxide film, a zirconium oxide film, a galliumoxide film, a tantalum oxide film, a magnesium oxide film, a lanthanumoxide film, a cerium oxide film, and a neodymium oxide film. Note thatinstead of a stacked structure of the insulating films 106 and 107, aninsulating film of a single layer formed using a material selected fromthe above or an insulating film of three or more layers may be used.

The insulating film 106 functions as a blocking film which keeps outoxygen. For example, in the case where excess oxygen is supplied to theinsulating film 107, the insulating film 114, the insulating film 116,and/or the oxide semiconductor film 108, the insulating film 106 cankeep out oxygen.

Note that the insulating film 107 that is in contact with the oxidesemiconductor film 108 functioning as a channel region of the transistor100 is preferably an oxide insulating film and preferably includes aregion including oxygen in excess of the stoichiometric composition(oxygen-excess region). In other words, the insulating film 107 is aninsulating film which is capable of releasing oxygen. In order toprovide the oxygen excess region in the insulating film 107, theinsulating film 107 is formed in an oxygen atmosphere, for example.Alternatively, the oxygen excess region may be formed by introduction ofoxygen into the insulating film 107 after the deposition. As a methodfor introducing oxygen, an ion implantation method, an ion dopingmethod, a plasma immersion ion implantation method, plasma treatment, orthe like may be employed.

In the case where hafnium oxide is used for the insulating film 107, thefollowing effect is attained. Hafnium oxide has a higher dielectricconstant than silicon oxide and silicon oxynitride. Therefore, by usinghafnium oxide or aluminum oxide, a physical thickness can be made largerthan an equivalent oxide thickness; thus, even in the case where theequivalent oxide thickness is less than or equal to 10 nm or less thanor equal to 5 nm, leakage current due to tunnel current can be low. Thatis, it is possible to provide a transistor with a low off-state current.Moreover, hafnium oxide with a crystalline structure has higherdielectric constant than hafnium oxide with an amorphous structure.Therefore, it is preferable to use hafnium oxide with a crystallinestructure in order to provide a transistor with a low off-state current.Examples of the crystalline structure include a monoclinic crystalstructure and a cubic crystal structure. Note that one embodiment of thepresent invention is not limited thereto.

In this embodiment, a silicon nitride film is formed as the insulatingfilm 106, and a silicon oxide film is formed as the insulating film 107.The silicon nitride film has a higher dielectric constant than a siliconoxide film and needs a larger thickness for capacitance equivalent tothat of the silicon oxide film. Thus, when the silicon nitride film isincluded in the gate insulating film of the transistor 150, the physicalthickness of the insulating film can be increased. This makes itpossible to reduce a decrease in withstand voltage of the transistor 100and furthermore to increase the withstand voltage, thereby reducingelectrostatic discharge damage to the transistor 100.

<Oxide Semiconductor Film>

The oxide semiconductor film 108 contains O, In, Zn, and M (M is Ti, Ga,Y, Zr, La, Ce, Nd, or Hf). Typically, In—Ga oxide, In—Zn oxide, orIn-M-Zn oxide can be used for the oxide semiconductor film 108. It isparticularly preferable to use In-M-Zn oxide for the semiconductor film108.

In the case where the oxide semiconductor film 108 is formed of In-M-Znoxide, it is preferable that the atomic ratio of metal elements of asputtering target used for forming the In-M-Zn oxide satisfy In≥M andZn≥M. As the atomic ratio of metal elements of such a sputtering target,In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, and In:M:Zn=3:1:2 are preferable. Notethat the atomic ratios of metal elements in the formed oxidesemiconductor film 108 vary from the above atomic ratio of metalelements of the sputtering target within a range of ±40% as an error.

Note that in the case where the oxide semiconductor film 108 is anIn-M-Zn oxide film, the proportion of In and the proportion of M, nottaking Zn and O into consideration, are preferably greater than or equalto 25 atomic % and less than 75 atomic %, respectively, furtherpreferably greater than or equal to 34 atomic % and less than 66 atomic%, respectively.

The energy gap of the oxide semiconductor film 108 is 2 eV or more,preferably 2.5 eV or more, further preferably 3 eV or more. With the useof an oxide semiconductor having such a wide energy gap, the off-statecurrent of the transistor 150 can be reduced.

The thickness of the oxide semiconductor film 108 is greater than orequal to 3 nm and less than or equal to 200 nm, preferably greater thanor equal to 3 nm and less than or equal to 100 nm, further preferablygreater than or equal to 3 nm and less than or equal to 50 nm.

An oxide semiconductor film with low carrier density is used as theoxide semiconductor film 108. For example, an oxide semiconductor filmwhose carrier density is lower than or equal to 1×10¹⁷/cm³, preferablylower than or equal to 1×10¹⁵/cm³, further preferably lower than orequal to 1×10¹³/cm³, still further preferably lower than or equal to1×10¹¹/cm³ is used as the oxide semiconductor film 108.

Note that, without limitation to the compositions and materialsdescribed above, a material with an appropriate composition may be useddepending on required semiconductor characteristics and electricalcharacteristics (e.g., field-effect mobility and threshold voltage) of atransistor. Further, in order to obtain required semiconductorcharacteristics of a transistor, it is preferable that the carrierdensity, the impurity concentration, the defect density, the atomicratio of a metal element to oxygen, the interatomic distance, thedensity, and the like of the oxide semiconductor film 108 be set to beappropriate.

Note that it is preferable to use, as the oxide semiconductor film 108,an oxide semiconductor film in which the impurity concentration is lowand density of defect states is low, in which case the transistor canhave more excellent electrical characteristics. Here, the state in whichimpurity concentration is low and density of defect states is low (theamount of oxygen vacancy is small) is referred to as “highly purifiedintrinsic” or “substantially highly purified intrinsic”. A highlypurified intrinsic or substantially highly purified intrinsic oxidesemiconductor film has few carrier generation sources, and thus can havea low carrier density. Thus, a transistor in which a channel region isformed in the oxide semiconductor film rarely has a negative thresholdvoltage (is rarely normally on). A highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor film has alow density of defect states and accordingly has few carrier traps insome cases. Further, the highly purified intrinsic or substantiallyhighly purified intrinsic oxide semiconductor film has an extremely lowoff-state current; even when an element has a channel width of 1×10⁶ μmand a channel length L of 10 μm, the off-state current can be less thanor equal to the measurement limit of a semiconductor parameter analyzer,i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drain voltage)between a source electrode and a drain electrode of from 1 V to 10 V.

Accordingly, the transistor in which the channel region is formed in thehighly purified intrinsic or substantially highly purified intrinsicoxide semiconductor film can have a small variation in electricalcharacteristics and high reliability. Charges trapped by the trap statesin the oxide semiconductor film take a long time to be released and maybehave like fixed charges. Thus, the transistor whose channel region isformed in the oxide semiconductor film having a high density of trapstates has unstable electrical characteristics in some cases. Asexamples of the impurities, hydrogen, nitrogen, alkali metal, alkalineearth metal, and the like are given.

Hydrogen included in the oxide semiconductor film reacts with oxygenbonded to a metal atom to be water, and also causes oxygen vacancy in alattice from which oxygen is released (or a portion from which oxygen isreleased). Due to entry of hydrogen into the oxygen vacancy, an electronserving as a carrier is generated in some cases. Furthermore, in somecases, bonding of part of hydrogen to oxygen bonded to a metal elementcauses generation of an electron serving as a carrier. Thus, atransistor including an oxide semiconductor film which contains hydrogenis likely to be normally on. Accordingly, it is preferable that hydrogenbe reduced as much as possible in the oxide semiconductor film 108.Specifically, in the oxide semiconductor film 108, the concentration ofhydrogen which is measured by SIMS is lower than or equal to 2×10²⁰atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, furtherpreferably lower than or equal to 1×10¹⁹ atoms/cm³, further preferablylower than or equal to 5×10¹⁸ atoms/cm³, further preferably lower thanor equal to 1×10¹⁸ atoms/cm³, further preferably lower than or equal to5×10¹⁷ atoms/cm³, further preferably lower than or equal to 1×10¹⁶atoms/cm³.

When silicon or carbon that is one of elements belonging to Group 14 isincluded in the oxide semiconductor film 108, oxygen vacancy isincreased in the oxide semiconductor film 108, and the oxidesemiconductor film 108 becomes an n-type film. Thus, the concentrationof silicon or carbon (the concentration is measured by SIMS) in theoxide semiconductor film 108 or the concentration of silicon or carbon(the concentration is measured by SIMS) in the vicinity of an interfacewith the oxide semiconductor film 108 is set to be lower than or equalto 2×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁷ atoms/cm³.

In addition, the concentration of alkali metal or alkaline earth metalof the oxide semiconductor film 108, which is measured by SIMS, is lowerthan or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generatecarriers when bonded to an oxide semiconductor, in which case theoff-state current of the transistor might be increased. Therefore, it ispreferable to reduce the concentration of alkali metal or alkaline earthmetal of the oxide semiconductor film 108.

Furthermore, when including nitrogen, the oxide semiconductor film 108easily becomes n-type by generation of electrons serving as carriers andan increase of carrier density. Thus, a transistor including an oxidesemiconductor film which contains nitrogen is likely to have normally-oncharacteristics. For this reason, nitrogen in the oxide semiconductorfilm is preferably reduced as much as possible; the concentration ofnitrogen which is measured by SIMS is preferably set, for example, lowerthan or equal to 5×10¹⁸ atoms/cm³.

The oxide semiconductor film 108 may have a non-single-crystalstructure, for example. The non-single crystal structure includes ac-axis aligned crystalline oxide semiconductor (CAAC-OS) which isdescribed later, a polycrystalline structure, a microcrystallinestructure described later, or an amorphous structure, for example. Amongthe non-single crystal structure, the amorphous structure has thehighest density of defect states, whereas CAAC-OS has the lowest densityof defect states.

The oxide semiconductor film 108 may have an amorphous structure, forexample. The oxide semiconductor films having the amorphous structureeach have disordered atomic arrangement and no crystalline component,for example. Alternatively, the oxide films having an amorphousstructure have, for example, an absolutely amorphous structure and nocrystal part.

Note that the oxide semiconductor film 108 may be a mixed film includingtwo or more of the following: a region having an amorphous structure, aregion having a microcrystalline structure, a region having apolycrystalline structure, a region of CAAC-OS, and a region having asingle-crystal structure. The mixed film has a single-layer structureincluding, for example, two or more of a region having an amorphousstructure, a region having a microcrystalline structure, a region havinga polycrystalline structure, a CAAC-OS region, and a region having asingle-crystal structure in some cases. Furthermore, in some cases, themixed film has a stacked-layer structure of two or more of a regionhaving an amorphous structure, a region having a microcrystallinestructure, a region having a polycrystalline structure, a CAAC-OSregion, and a region having a single-crystal structure.

<Protective Insulating Film>

The insulating films 114, 116, and 118 function as protective insulatingfilms. The insulating films 114 and 116 contains oxygen. Furthermore,the insulating film 114 is an insulating film which is permeable tooxygen. Note that the insulating film 114 also functions as a film whichrelieves damage to the oxide semiconductor film 108 at the time offorming the insulating film 116 in a later step.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 5 nm and less than or equal to 150nm, preferably greater than or equal to 5 nm and less than or equal to50 nm can be used as the insulating film 114.

In addition, it is preferable that the number of defects in theinsulating film 114 be small and typically, the spin densitycorresponding to a signal that appears at g=2.001 due to a dangling bondof silicon be lower than or equal to 3×10¹⁷ spins/cm³ by electron spinresonance (ESR) measurement. This is because if the density of defectsin the insulating film 114 is high, oxygen is bonded to the defects andthe amount of oxygen that penetrates the insulating film 114 isdecreased.

Note that all oxygen entering the insulating film 114 from the outsidedoes not move to the outside of the insulating film 114 and some oxygenremains in the insulating film 114. Furthermore, movement of oxygenoccurs in the insulating film 114 in some cases in such a manner thatoxygen enters the insulating film 114 and oxygen included in theinsulating film 114 moves to the outside of the insulating film 114.When an oxide insulating film which is permeable to oxygen is formed asthe insulating film 114, oxygen released from the insulating film 116provided over the insulating film 114 can be moved to the oxidesemiconductor film 108 through the insulating film 114.

The insulating film 116 is formed using an oxide insulating film thatcontains oxygen in excess of that in the stoichiometric composition.Part of oxygen is released by heating from the oxide insulating filmincluding oxygen in excess of that in the stoichiometric composition.The oxide insulating film including oxygen in excess of that in thestoichiometric composition is an oxide insulating film of which theamount of released oxygen converted into oxygen molecules is greaterthan or equal to 1.0×10¹⁹/cm³, preferably greater than or equal to3.0×10²⁰/cm³ in TDS analysis. Note that the temperature of the filmsurface in the TDS analysis is preferably higher than or equal to 100°C. and lower than or equal to 700° C., or higher than or equal to 100°C. and lower than or equal to 500° C.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, preferably greater than or equal to 50 nm and less than or equal to400 nm can be used as the insulating film 116.

It is preferable that the number of defects in the insulating film 116be small, and typically the spin density corresponding to a signal whichappears at g=2.001 due to a dangling bond of silicon, be lower than1.5×10¹⁸ spins/cm³, more preferably lower than or equal to 1×10¹⁸spins/cm³ by ESR measurement. Note that the insulating film 116 isprovided more apart from the oxide semiconductor film 108 than theinsulating film 114 is; thus, the insulating film 116 may have higherdensity of defects than the insulating film 114.

Furthermore, the insulating films 114 and 116 can be formed usinginsulating films formed of the same kinds of materials; thus, a boundarybetween the insulating films 114 and 116 cannot be clearly observed insome cases. Thus, in this embodiment, the boundary between theinsulating films 114 and 116 is shown by a dashed line. Although atwo-layer structure of the insulating films 114 and 116 is described inthis embodiment, the present invention is not limited to this. Forexample, a single-layer structure of the insulating film 114 may beused.

The insulating film 118 contains nitrogen. Alternatively, the insulatingfilm 118 contains nitrogen and silicon. The insulating film 118 has afunction of blocking oxygen, hydrogen, water, an alkali metal, analkaline earth metal, or the like. It is possible to prevent outwarddiffusion of oxygen from the oxide semiconductor film 108, outwarddiffusion of oxygen included in the insulating films 114 and 116, andentry of hydrogen, water, or the like into the oxide semiconductor film108 from the outside by providing the insulating film 118. A nitrideinsulating film, for example, can be used as the insulating film 118.The nitride insulating film is formed using silicon nitride, siliconnitride oxide, aluminum nitride, aluminum nitride oxide, or the like.Note that instead of the nitride insulating film having a blockingeffect against oxygen, hydrogen, water, alkali metal, alkaline earthmetal, and the like, an oxide insulating film having a blocking effectagainst oxygen, hydrogen, water, and the like, may be provided. As theoxide insulating film having a blocking effect against oxygen, hydrogen,water, and the like, an aluminum oxide film, an aluminum oxynitridefilm, a gallium oxide film, a gallium oxynitride film, an yttrium oxidefilm, an yttrium oxynitride film, a hafnium oxide film, and a hafniumoxynitride film can be given.

Although the variety of films such as the conductive films, theinsulating films, and the oxide semiconductor films which are describedabove can be formed by a sputtering method or a PECVD method, such filmsmay be formed by another method, e.g., an atomic layer deposition (ALD)method or a thermal CVD method. As an example of a thermal CVD method, ametal organic chemical vapor deposition (MOCVD) method can be given.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to the chamber at a timeso that the pressure in a chamber is set to an atmospheric pressure or areduced pressure, and react with each other in the vicinity of thesubstrate or over the substrate.

Deposition by an ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). For example, a first source gas is introduced, aninert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first gas so that thesource gases are not mixed, and then a second source gas is introduced.Note that in the case where the first source gas and the inert gas areintroduced at a time, the inert gas serves as a carrier gas, and theinert gas may also be introduced at the same time as the introduction ofthe second source gas. Alternatively, the first source gas may beexhausted by vacuum evacuation instead of the introduction of the inertgas, and then the second source gas may be introduced. The first sourcegas is adsorbed on the surface of the substrate to form a first layer;then the second source gas is introduced to react with the first layer;as a result, a second layer is stacked over the first layer, so that athin film is formed. The sequence of the gas introduction is repeatedplural times until a desired thickness is obtained, whereby a thin filmwith excellent step coverage can be formed. The thickness of the thinfilm can be adjusted by the number of repetition times of the sequenceof the gas introduction; therefore, an ALD method makes it possible toaccurately adjust a thickness and thus is suitable for manufacturing aminute FET.

The variety of films such as the conductive films, the insulating films,the oxide semiconductor films, and the metal oxide films in thisembodiment can be formed by a thermal CVD method such as an MOCVDmethod. For example, in the case where an In—Ga—Zn—O film is formed,trimethylindium, trimethylgallium, and dimethylzinc are used. Note thatthe chemical formula of trimethylindium is In(CH₃)₃. The chemicalformula of trimethylgallium is Ga(CH₃)₃. The chemical formula ofdimethylzinc is Zn(CH₃)₂. Without limitation to the above combination,triethylgallium (chemical formula: Ga(C₂H₅)₃) can be used instead oftrimethylgallium and diethylzinc (chemical formula: Zn(C₂H₅)₂) can beused instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed by adeposition apparatus using an ALD method, two kinds of gases, i.e.,ozone (O₃) as an oxidizer and a source gas which is obtained byvaporizing liquid containing a solvent and a hafnium precursor compound(a hafnium alkoxide solution, typically tetrakis(dimethylamide)hafnium(TDMAH)) are used. Note that the chemical formula oftetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples of anothermaterial liquid include tetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed by adeposition apparatus using an ALD method, two kinds of gases, e.g., H₂Oas an oxidizer and a source gas which is obtained by vaporizing liquidcontaining a solvent and an aluminum precursor compound (e.g.,trimethylaluminum (TMA)) are used. Note that the chemical formula oftrimethylaluminum is Al(CH₃)₃. Examples of another material liquidinclude tris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed by adeposition apparatus using an ALD method, hexachlorodisilane is adsorbedon a surface where a film is to be formed, chlorine included in theadsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ ordinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is fainted using adeposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas aresequentially introduced plural times to form an initial tungsten film,and then a WF₆ gas and an H₂ gas are introduced at a time, so that atungsten film is formed. Note that an SiH₄ gas may be used instead of aB₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., anIn—Ga—Zn—O film is formed using a deposition apparatus employing ALD, anIn(CH₃)₃ gas and an O₃ gas are sequentially introduced plural times toform an InO₂ layer, a Ga(CH₃)₃ gas and an O₃ gas are introduced at atime to form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas areintroduced at a time to form a ZnO layer. Note that the order of theselayers is not limited to this example. A mixed compound layer such as anIn—Ga—O layer, an In—Zn—O layer, or a Ga—Zn—O layer may be formed bymixing of these gases. Note that although an H₂O gas which is obtainedby bubbling with an inert gas such as Ar may be used instead of an O₃gas, it is preferable to use an O₃ gas, which does not contain H.Further, instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may be used.Instead of a Ga(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used. Furthermore, aZn(CH₃)₂ gas may be used.

Structure examples different from that of the transistor 100 in FIGS. 1Ato 1C are described with reference to FIGS. 2A to 2D. Note that in thecase where a portion has a function similar to that described above, thesame hatch pattern is applied to the portion, and the portion is notespecially denoted by a reference numeral in some cases.

<Structure Example 2 of Semiconductor Device>

FIG. 2A is a cross-sectional view in the channel length direction of atransistor 100A and FIG. 2B is a cross-sectional view in the channelwidth direction of the transistor 100A. FIG. 2C is a cross-sectionalview in the channel length direction of a transistor 100B and FIG. 2D isa cross-sectional view in the channel width direction of the transistor100B. Note that top views of the transistor 100A and the transistor 100Bare omitted here because they are similar to the top view of FIG. 1A.

The transistor 100A illustrated in FIGS. 2A and 2B includes theconductive film 104 functioning as a gate electrode over the substrate102, the insulating film 106 over the substrate 102 and the conductivefilm 104, the insulating film 107 over the insulating film 106, theoxide semiconductor film 108 over the insulating film 107, and theconductive films 112 a and 112 b functioning as source and drainelectrodes electrically connected to the oxide semiconductor film 108.Over the transistor 100A, specifically, over the conductive films 112 aand 112 b and the oxide semiconductor film 108, the insulating films114, 116, and 118 and an insulating film 131 (also referred to as afifth insulating film) are provided. The insulating films 114, 116, 118,and 131 function as protective insulating films for the transistor 100A.

The transistor 100A is different from the transistor 100 in FIGS. 1B and1C in that the insulating film 131 is provided. Specifically, theinsulating film 131 is provided between the insulating film 116 and theinsulating film 118.

The transistor 100B illustrated in FIGS. 2C and 2D includes theconductive film 104 functioning as a gate electrode over the substrate102, the insulating film 106 over the substrate 102 and the conductivefilm 104, the insulating film 107 over the insulating film 106, theoxide semiconductor film 108 over the insulating film 107, and theconductive films 112 a and 112 b functioning as source and drainelectrodes electrically connected to the oxide semiconductor film 108.Over the transistor 100B, specifically, over the conductive films 112 aand 112 b and the oxide semiconductor film 108, the insulating films114, 116, 118, and 131 are provided. The insulating films 114, 116, 118,and 131 function as protective insulating films for the transistor 100B.

The transistor 100B is different from the transistor 100 in FIGS. 1B and1C in that the insulating film 131 is provided. Specifically, theinsulating film 131 is provided between the insulating film 114 and theinsulating film 116.

The insulating film 131 has a function of inhibiting release of oxygenincluded in the insulating film 114 and/or the insulating film 116.Furthermore, the insulating film 131 is formed of oxide or nitride ofmetal, and the metal includes at least one selected from indium, zinc,titanium, aluminum, tungsten, tantalum, and molybdenum.

The insulating film 131 can inhibit oxygen included in the insulatingfilm 114 and/or the insulating film 116 from diffusing to the outside.In other words, the insulating film 131 is provided, whereby oxygenincluded in the insulating film 114 and/or the insulating film 116 canbe favorably moved to the oxide semiconductor film 108 side. Thus,oxygen vacancy in the oxide semiconductor film 108 is filled, whereby ahighly reliable semiconductor device can be provided.

A structure example different from that of the transistor 100 in FIGS.1A to 1C is described with reference to FIGS. 3A to 3C. Note that in thecase where a portion has a function similar to that described above, thesame hatch pattern is applied to the portion, and the portion is notespecially denoted by a reference numeral in some cases.

<Structure Example 3 of Semiconductor Device>

FIG. 3A is a top view of a transistor 150 that is a semiconductor deviceof one embodiment of the present invention. FIG. 3B is a cross-sectionalview taken along dashed-dotted line X1-X2 illustrated in FIG. 3A, andFIG. 3C is a cross-sectional view taken along dashed-dotted line Y1-Y2illustrated in FIG. 3A.

The transistor 150 includes the conductive film 104 functioning as agate electrode over the substrate 102, the insulating film 106 over thesubstrate 102 and the conductive film 104, the insulating film 107 overthe insulating film 106, the oxide semiconductor film 108 over theinsulating film 107, the insulating film 114 over the oxidesemiconductor film 108, the insulating film 116 over the insulating film114, and the conductive films 112 a and 112 b functioning as source anddrain electrodes electrically connected to the oxide semiconductor film108 though openings 141 a and 141 b provided in the insulating film 114and the insulating film 116. Over the transistor 150, specifically, overthe conductive films 112 a and 112 b and the insulating film 116, theinsulating film 118 is provided. The insulating film 114 and theinsulating film 116 function as protective insulating films for theoxide semiconductor film 108. The insulating film 118 functions as aprotective insulating film for the transistor 150.

Although the transistors 100, 100A, and 100B each have a channel-etchedstructure, the transistor 150 in FIGS. 3A to 3C has a channel-protectivestructure. Thus, either the channel-etched structure or thechannel-protective structure can be applied to the semiconductor deviceof one embodiment of the present invention.

Like the transistor 100, the transistor 150 is provided with theinsulating film 114 over the oxide semiconductor film 108; therefore,oxygen included in the insulating film 114 can fill oxygen vacancy inthe oxide semiconductor film 108.

A structure example different from that of the transistor 150 in FIGS.3A to 3C is described with reference to FIGS. 4A to 4D. Note that in thecase where a portion has a function similar to that described above, thesame hatch pattern is applied to the portion, and the portion is notespecially denoted by a reference numeral in some cases.

<Structure Example 4 of Semiconductor Device>

FIG. 4A is a cross-sectional view in the channel length direction of atransistor 150A and FIG. 4B is a cross-sectional view in the channelwidth direction of the transistor 150A. FIG. 4C is a cross-sectionalview in the channel length direction of a transistor 150B and FIG. 4D isa cross-sectional view in the channel width direction of the transistor150B. Note that top views of the transistor 150A and the transistor 150Bare omitted here because they are similar to the top view of FIG. 3A.

The transistor 150A illustrated in FIGS. 4A and 4B includes theconductive film 104 functioning as a gate electrode over the substrate102, the insulating film 106 over the substrate 102 and the conductivefilm 104, the insulating film 107 over the insulating film 106, theoxide semiconductor film 108 over the insulating film 107, theinsulating film 114 over the oxide semiconductor film 108, theinsulating film 116 over the insulating film 114, the insulating film131 over the insulating film 116, and the conductive films 112 a and 112b functioning as source and drain electrodes electrically connected tothe oxide semiconductor film 108 though the openings 141 a and 141 bprovided in the insulating films 114, 116, and 131. Over the transistor150A, specifically, over the conductive films 112 a and 112 b and theinsulating film 131, the insulating film 118 is provided. The insulatingfilms 114, 116, and 131 function as protective insulating films for theoxide semiconductor film 108. The insulating film 118 functions as aprotective insulating film for the transistor 150A.

The transistor 150A is different from the transistor 150 in FIGS. 3B and3C in that the insulating film 131 is provided. Specifically, theinsulating film 131 is provided between the insulating film 116 and theinsulating film 118. The other components are the same as those of thetransistor 150, and the effect similar to that in the case of thetransistor 150 is obtained.

The transistor 150B illustrated in FIGS. 4C and 4D includes theconductive film 104 functioning as a gate electrode over the substrate102, the insulating film 106 over the substrate 102 and the conductivefilm 104, the insulating film 107 over the insulating film 106, theoxide semiconductor film 108 over the insulating film 107, theinsulating film 114 over the oxide semiconductor film 108, theinsulating film 131 over the insulating film 114, the insulating film116 over the insulating film 131, and the conductive films 112 a and 112b functioning as source and drain electrodes electrically connected tothe oxide semiconductor film 108 though the openings 141 a and 141 bprovided in the insulating films 114, 116, and 131. Over the transistor150B, specifically, over the conductive films 112 a and 112 b and theinsulating film 116, the insulating film 118 is provided. The insulatingfilms 114, 116, and 131 function as protective insulating films for theoxide semiconductor film 108. The insulating film 118 functions as aprotective insulating film for the transistor 150B.

The transistor 150B is different from the transistor 150 in FIGS. 3B and3C in that the insulating film 131 is provided. Specifically, theinsulating film 131 is provided between the insulating film 114 and theinsulating film 116. The other components are the same as those of thetransistor 150, and the effect similar to that in the case of thetransistor 150 is obtained.

A structure example different from that of the transistor 150 in FIGS.3A to 3C is described with reference to FIGS. 5A to 5C. Note that in thecase where a portion has a function similar to that described above, thesame hatch pattern is applied to the portion, and the portion is notespecially denoted by a reference numeral in some cases.

<Structural Example 5 of Semiconductor Device>

FIG. 5A is a top view of a transistor 160 that is a semiconductor deviceof one embodiment of the present invention. FIG. 5B is a cross-sectionalview taken along a dashed dotted line X1-X2 in FIG. 5A, and FIG. 5C is across-sectional view taken along a dashed dotted line Y1-Y2 in FIG. 5A.

The transistor 160 includes the conductive film 104 functioning as agate electrode over the substrate 102, the insulating film 106 over thesubstrate 102 and the conductive film 104, the insulating film 107 overthe insulating film 106, the oxide semiconductor film 108 over theinsulating film 107, the insulating film 114 over the oxidesemiconductor film 108, the insulating film 116 over the insulating film114, and the conductive films 112 a and 112 b functioning as source anddrain electrodes electrically connected to the oxide semiconductor film108. Over the transistor 160, specifically, over the conductive films112 a and 112 b and the insulating film 116, the insulating film 118 isprovided. The insulating films 114 and 116 function as protectiveinsulating films for the oxide semiconductor film 108. The insulatingfilm 118 functions as a protective insulating film for the transistor160.

The transistor 160 is different from the transistor 150 in FIGS. 3A to3C in the shapes of the insulating films 114 and 116. Specifically, theinsulating films 114 and 116 of the transistor 160 have island shapesand are provided over a channel region of the oxide semiconductor film108. The other components are the same as those of the transistor 150,and the effect similar to that in the case of the transistor 150 isobtained.

A structure example different from that of the transistor 160 in FIGS.5A to 5C is described with reference to FIGS. 6A to 6D. Note that in thecase where a portion has a function similar to that described above, thesame hatch pattern is applied to the portion, and the portion is notespecially denoted by a reference numeral in some cases.

<Structure Example 6 of Semiconductor Device>

FIG. 6A is a cross-sectional view in the channel length direction of atransistor 160A and FIG. 6B is a cross-sectional view in the channelwidth direction of the transistor 160A. FIG. 6C is a cross-sectionalview in the channel length direction of a transistor 160B and FIG. 6D isa cross-sectional view in the channel width direction of the transistor160B. Note that top views of the transistor 160A and the transistor 160Bare omitted here because they are similar to the top view of FIG. 5A.

The transistor 160A includes the conductive film 104 functioning as agate electrode over the substrate 102, the insulating film 106 over thesubstrate 102 and the conductive film 104, the insulating film 107 overthe insulating film 106, the oxide semiconductor film 108 over theinsulating film 107, the insulating film 114 over the oxidesemiconductor film 108, the insulating film 116 over the insulating film114, the insulating film 131 over the insulating film 116, and theconductive films 112 a and 112 b functioning as source and drainelectrodes electrically connected to the oxide semiconductor film 108.Over the transistor 160A, specifically, over the conductive films 112 aand 112 b and the insulating film 131, the insulating film 118 isprovided. The insulating films 114, 116, and 131 function as protectiveinsulating films for the oxide semiconductor film 108. The insulatingfilm 118 functions as a protective insulating film for the transistor160A.

The transistor 160A is different from the transistor 160 in FIGS. 5B and5C in that the insulating film 131 is provided. Specifically, theinsulating film 131 of the transistor 160A is provided between theinsulating film 116 and the insulating film 118. The other componentsare the same as those of the transistor 160, and the effect similar tothat in the case of the transistor 160 is obtained.

The transistor 160B includes the conductive film 104 functioning as agate electrode over the substrate 102, the insulating film 106 over thesubstrate 102 and the conductive film 104, the insulating film 107 overthe insulating film 106, the oxide semiconductor film 108 over theinsulating film 107, the insulating film 114 over the oxidesemiconductor film 108, the insulating film 131 over the insulating film114, the insulating film 116 over the insulating film 131, and theconductive films 112 a and 112 b functioning as source and drainelectrodes electrically connected to the oxide semiconductor film 108.Over the transistor 160B, specifically, over the conductive films 112 aand 112 b and the insulating film 116, the insulating film 118 isprovided. The insulating films 114, 116, and 131 function as protectiveinsulating films for the oxide semiconductor film 108. The insulatingfilm 118 functions as a protective insulating film for the transistor160B.

The transistor 160B is different from the transistor 160 in FIGS. 5B and5C in that the insulating film 131 is provided. Specifically, theinsulating film 131 of the transistor 160B is provided between theinsulating film 114 and the insulating film 116. The other componentsare the same as those of the transistor 160, and the effect similar tothat in the case of the transistor 160 is obtained.

A structure example different from that of the transistor 100 in FIGS.1A to 1C is described with reference to FIGS. 7A to 7C. Note that in thecase where a portion has a function similar to that described above, thesame hatch pattern is applied to the portion, and the portion is notespecially denoted by a reference numeral in some cases.

<Structural Example 7 of Semiconductor Device>

FIG. 7A is a top view of a transistor 170 that is a semiconductor deviceof one embodiment of the present invention. FIG. 7B is a cross-sectionalview taken along a dashed dotted line X1-X2 in FIG. 7A, and FIG. 7C is across-sectional view taken along a dashed dotted line Y1-Y2 in FIG. 7A.

The transistor 170 includes the conductive film 104 functioning as agate electrode over the substrate 102, the insulating film 106 over thesubstrate 102 and the conductive film 104, the insulating film 107 overthe insulating film 106, the oxide semiconductor film 108 over theinsulating film 107, the insulating film 114 over the oxidesemiconductor film 108, the insulating film 116 over the insulating film114, and the conductive films 112 a and 112 b functioning as source anddrain electrodes electrically connected to the oxide semiconductor film108. Over the transistor 170, specifically, over the conductive films112 a and 112 b and the insulating film 116, the insulating film 118 andconductive films 120 a and 120 b are provided. The insulating films 114and 116 function as protective insulating films for the oxidesemiconductor film 108. The insulating film 118 functions as aprotective insulating film for the transistor 170. The conductive film120 a is connected to the conductive film 112 b through an opening 142 cprovided in the insulating films 114, 116, and 118. The conductive film120 b is formed over the insulating film 118 to overlap the oxidesemiconductor film 108.

The insulating films 114, 116, and 118 in the transistor 170 function assecond gate insulating films of the transistor 170. The conductive film120 a in the transistor 170 functions as, for example, a pixel electrodeused for a display device. The conductive film 120 b in the transistor170 functions as a second gate electrode (also referred to as a backgate electrode).

As illustrated in FIG. 7C, the conductive film 120 b is connected to theconductive film 104 functioning as a gate electrode through openings 142a and 142 b provided in the insulating films 106, 107, 114, 116, and118. Accordingly, the conductive film 120 b and the conductive film 104are supplied with the same potential.

Note that although the structure in which the openings 142 a and 142 bare provided so that the conductive film 120 b and the conductive film104 are connected to each other is described in this embodiment, oneembodiment of the present invention is not limited thereto. For example,a structure in which only one of the openings 142 a and 142 b isprovided so that the conductive film 120 b and the conductive film 104are connected to each other, or a structure in which the openings 142 aand 142 b are not provided and the conductive film 120 b and theconductive film 104 are not connected to each other may be employed.Note that in the case where the conductive film 120 b and the conductivefilm 104 are not connected to each other, it is possible to applydifferent potentials to the conductive film 120 b and the conductivefilm 104.

As illustrated in FIG. 7B, the oxide semiconductor film 108 ispositioned to face each of the conductive film 104 functioning as a gateelectrode and the conductive film 120 b functioning as a second gateelectrode, and is sandwiched between the two conductive filmsfunctioning as gate electrodes. The lengths in the channel lengthdirection and the channel width direction of the conductive film 120 bfunctioning as a second gate electrode are longer than those in thechannel length direction and the channel width direction of the oxidesemiconductor film 108. The whole oxide semiconductor film 108 iscovered with the conductive film 120 b with the insulating films 114,116, and 118 positioned therebetween. Since the conductive film 120 bfunctioning as a second gate electrode is connected to the conductivefilm 104 functioning as a gate electrode through the opening 142 a and142 b provided in the insulating films 106, 107, 114, 116, and 118, aside surface of the oxide semiconductor film 108 in the channel widthdirection faces the conductive film 120 b functioning as a second gateelectrode with the insulating films 114, 116, and 118 positionedtherebetween.

In other words, in the channel width direction of the transistor 170,the conductive film 104 functioning as a gate electrode and theconductive film 120 b functioning as a second gate electrode areconnected to each other through the openings provided in the insulatingfilms 106 and 107 functioning as gate insulating films, and theinsulating films 114, 116, and 118 functioning as second gate insulatingfilms; and the conductive film 104 and the conductive film 120 bsurround the oxide semiconductor film 108 with the insulating films 106and 107 functioning as gate insulating films, and the insulating films114, 116, and 118 functioning as second gate insulating films positionedtherebetween.

Such a structure makes it possible that the oxide semiconductor film 108included in the transistor 170 is electrically surrounded by electricfields of the conductive film 104 functioning as a gate electrode andthe conductive film 120 b functioning as a second gate electrode. Adevice structure of a transistor, like that of the transistor 170, inwhich electric fields of a gate electrode and a second gate electrodeelectrically surround an oxide semiconductor film where a channel regionis formed can be referred to as a surrounded channel (s-channel)structure.

Since the transistor 170 has the s-channel structure, an electric fieldfor inducing a channel can be effectively applied to the oxidesemiconductor film 108 by the conductive film 104 functioning as a gateelectrode; therefore, the current drive capability of the transistor 170can be improved and high on-state current characteristics can beobtained. Since the on-state current can be increased, it is possible toreduce the size of the transistor 170. In addition, since the transistor170 is surrounded by the conductive film 104 functioning as a gateelectrode and the conductive film 120 b functioning as a second gateelectrode, the mechanical strength of the transistor 170 can beincreased.

Structure examples different from that of the transistor 100 in FIGS. 1Ato 1C are described with reference to FIGS. 8A to 8D. Note that in thecase where a portion has a function similar to that described above, thesame hatch pattern is applied to the portion, and the portion is notespecially denoted by a reference numeral in some cases.

<Structure Example 8 of Semiconductor Device>

FIGS. 8A and 8B each illustrate a cross-sectional view of a modificationexample of the transistor 100 in FIGS. 1B and 1C. FIGS. 8C and 8D eachillustrate a cross-sectional view of another modification example of thetransistor 100 in FIGS. 1B and 1C.

A transistor 100C in FIGS. 8A and 8B has the same structure as thetransistor 100 in FIGS. 1B and 1C except that the oxide semiconductorfilm 108 has a three-layer structure. Specifically, the oxidesemiconductor film 108 of the transistor 100C includes an oxidesemiconductor film 108 a, an oxide semiconductor film 108 b, and anoxide semiconductor film 108 c.

A transistor 100D in FIGS. 8C and 8D has the same structure as thetransistor 100 in FIGS. 1B and 1C except that the oxide semiconductorfilm 108 has a two-layer structure. Specifically, the oxidesemiconductor film 108 of the transistor 100D includes the oxidesemiconductor film 108 a and the oxide semiconductor film 108 b.

Here, a band structure including the oxide semiconductor films 108 a,108 b, and 108 c and insulating films in contact with the oxidesemiconductor film 108 is described with reference to FIGS. 9A and 9B.

FIG. 9A shows an example of a band structure in the thickness directionof a stack including the insulating film 107, the oxide semiconductorfilms 108 a, 108 b, and 108 c, and the insulating film 114. FIG. 9Bshows an example of a band structure in the thickness direction of astack including the insulating film 107, the oxide semiconductor films108 a and 108 b, and the insulating film 114. For easy understanding,the conduction band minimum (Ec) of each of the insulating film 107, theoxide semiconductor films 108 a, 108 b, and 108 c, and the insulatingfilm 114 is shown in the band structures.

In FIG. 9A, a silicon oxide film is used as each of the insulating films107 and 114, an oxide semiconductor film formed using a metal oxidetarget having an atomic ratio of metal elements of In:Ga:Zn=1:1:1 isused as the oxide semiconductor film 108 a, an oxide semiconductor filmformed using a metal oxide target having an atomic ratio of metalelements of In:Ga:Zn=1:4:5 is used as the oxide semiconductor film 108b, and an oxide semiconductor film formed using a metal oxide targethaving an atomic ratio of metal elements of In:Ga:Zn=1:3:6 is used asthe oxide semiconductor film 108 c.

In the band structure of FIG. 9B, a silicon oxide film is used as eachof the insulating films 107 and 114, an oxide semiconductor film formedusing a metal oxide target having an atomic ratio of metal elements ofIn:Ga:Zn=1:1:1 is used as the oxide semiconductor film 108 a, and ametal oxide film formed using a metal oxide target having an atomicratio of metal elements of In:Ga:Zn=1:3:6 is used as the oxidesemiconductor film 108 b.

As illustrated in FIGS. 9A and 9B, the conduction band minimum smoothlyvaries between the oxide semiconductor film 108 a and the oxidesemiconductor film 108 b. In other words, the conduction band minimum iscontinuously varied or continuously connected. To obtain such a bandstructure, it is preferable that there exist no impurity, which forms adefect state such as a trap center or a recombination center for theoxide semiconductor, at the interface between the oxide semiconductorfilm 108 a and the oxide semiconductor film 108 b.

To form a continuous junction between the oxide semiconductor film 108 aand the oxide semiconductor film 108 b, it is necessary to form thefilms successively without exposure to the air by using a multi-chamberdeposition apparatus (sputtering apparatus) provided with a load lockchamber.

With the band structure of FIG. 9A or FIG. 9B, the oxide semiconductorfilm 108 a serves as a well, and a channel region is formed in the oxidesemiconductor film 108 a in the transistor with the stacked-layerstructure.

By providing the oxide semiconductor film 108 b and/or the oxidesemiconductor film 108 c, the oxide semiconductor film 108 a can bedistanced away from trap states.

In addition, the trap states might be more distant from the vacuum levelthan the conduction band minimum (Ec) of the oxide semiconductor film108 a functioning as a channel region, so that electrons are likely tobe accumulated in the trap states. When the electrons are accumulated inthe trap states, the electrons become negative fixed electric charge, sothat the threshold voltage of the transistor is shifted in the positivedirection. Therefore, it is preferable that the trap states be closer tothe vacuum level than the conduction band minimum (Ec) of the oxidesemiconductor film 108 a. Such a structure inhibits accumulation ofelectrons in the trap states. As a result, the on-state current and thefield-effect mobility of the transistor can be increased.

In FIGS. 9A and 9B, the conduction band minimum of each of the oxidesemiconductor films 108 b and 108 c is closer to the vacuum level thanthat of the oxide semiconductor film 108 a. Typically, an energydifference between the conduction band minimum of the oxidesemiconductor film 108 a and the conduction band minimum of each of theoxide semiconductor films 108 b and 108 c is greater than or equal to0.15 eV or greater than or equal to 0.5 eV, and less than or equal to 2eV or less than or equal to 1 eV. That is, the difference between theelectron affinity of each of the oxide semiconductor films 108 b and 108c and the electron affinity of the oxide semiconductor film 108 a isgreater than or equal to 0.15 eV or greater than or equal to 0.5 eV, andless than or equal to 2 eV or less than or equal to 1 eV.

In such a structure, the oxide semiconductor film 108 a serves as a mainpath of current and functions as a channel region. In addition, sincethe oxide semiconductor films 108 b and 108 c each include one or moremetal elements included in the oxide semiconductor film 108 a in which achannel region is formed, interface scattering is less likely to occurat the interface between the oxide semiconductor film 108 a and theoxide semiconductor film 108 b. Thus, the transistor can have highfield-effect mobility because the movement of carriers is not hinderedat the interface.

To prevent each of the oxide semiconductor films 108 b and 108 c fromfunctioning as part of a channel region, a material having sufficientlylow conductivity is used for the oxide semiconductor films 108 b and 108c. Alternatively, a material which has a smaller electron affinity (adifference in energy level between the vacuum level and the conductionband minimum) than the oxide semiconductor film 108 a and has adifference in the conduction band minimum from the oxide semiconductorfilm 108 a (band offset) is used for the oxide semiconductor films 108 band 108 c. Furthermore, to inhibit generation of a difference betweenthreshold voltages due to the value of the drain voltage, it ispreferable to form the oxide semiconductor films 108 b and 108 c using amaterial whose conduction band minimum is closer to the vacuum levelthan that of the oxide semiconductor film 108 a is by more than 0.2 eV,preferably 0.5 eV or more.

It is preferable that the oxide semiconductor films 108 b and 108 c nothave a spinel crystal structure. This is because if the oxidesemiconductor films 108 b and 108 c have a spinel crystal structure, aconstituent element of the conductive films 112 a and 112 b might bediffused into the oxide semiconductor film 108 a at the interfacebetween the spinel crystal structure and another region. Note that eachof the oxide semiconductor film 108 b and 108 c is preferably a CAAC-OS,which is described later, in which case a higher blocking propertyagainst constituent elements of the conductive films 112 a and 112 b,e.g., copper elements, is obtained.

The thickness of each of the oxide semiconductor films 108 b and 108 cis greater than or equal to a thickness that is capable of inhibitingdiffusion of the constituent element of the conductive films 112 a and112 b into the oxide semiconductor film 108 a, and less than a thicknessthat inhibits supply of oxygen from the insulating film 114 to the oxidesemiconductor film 108 a. For example, when the thickness of each of theoxide semiconductor films 108 b and 108 c is greater than or equal to 10nm, the constituent elements of the conductive films 112 a and 112 b canbe prevented from diffusing into the oxide semiconductor film 108 a.When the thickness of each of the oxide semiconductor films 108 b and108 c is less than or equal to 100 nm, oxygen can be effectivelysupplied from the insulating films 114 and 116 to the oxidesemiconductor film 108 a.

When the oxide semiconductor films 108 b and 108 c are each an In-M-Znoxide in which the atomic ratio of the element M (M is Ti, Ga, Y, Zr,La, Ce, Nd, Sn, or Hf) is higher than that of In, the energy gap of eachof the oxide semiconductor films 108 b and 108 c can be large and theelectron affinity thereof can be small. Therefore, a difference inelectron affinity between the oxide semiconductor film 108 a and each ofthe oxide semiconductor films 108 b and 108 c may be controlled by theproportion of the element M. Furthermore, oxygen vacancy is less likelyto be generated in the oxide semiconductor film in which the atomicratio of Ti, Ga, Y, Zr, La, Ce, Nd, Sn, or Hf is higher than that of Inbecause Ti, Ga, Y, Zr, La, Ce, Nd, Sn, and Hf each are a metal elementthat is strongly bonded to oxygen.

When an In-M-Zn oxide is used for the oxide semiconductor films 108 band 108 c, the proportions of In and M, not taking Zn and O intoconsideration, is preferably as follows: the atomic percentage of In isless than 50 atomic % and the atomic percentage of M is greater than orequal to 50 atomic %; further preferably, the atomic percentage of In isless than 25 atomic % and the atomic percentage of M is greater than orequal to 75 atomic %. Alternatively, a gallium oxide film may be used aseach of the oxide semiconductor films 108 b and 108 c.

Furthermore, in the case where each of the oxide semiconductor films 108a, 108 b, and 108 c is an In-M-Zn oxide, the proportion of M in each ofthe oxide semiconductor films 108 b and 108 c is higher than that in theoxide semiconductor film 108 a. Typically, the proportion of M in eachof the oxide semiconductor films 108 b and 108 c is 1.5 or more times,preferably twice or more, more preferably three or more times that inthe oxide semiconductor film 108 a.

Furthermore, in the case where the oxide semiconductor films 108 a, 108b, and 108 c are each an In-M-Zn oxide, when the oxide semiconductorfilm 108 a has an atomic ratio of In:M:Zn=x₁:y₁:z₁ and the oxidesemiconductor films 108 b and 108 c each have an atomic ratio ofIn:M:Zn=x₂:y₂:z₂, y₂/x₂ is larger than y₁/x₁, preferably y₂/x₂ is 1.5 ormore times as large as y₁/x₁, further preferably, y₂/x₂ is two or moretimes as large as y₁/x₁, still further preferably y₂/x₂ is three or moretimes or four or more times as large as y₁/x₁. At this time, y₁ ispreferably greater than or equal to x₁ in the oxide semiconductor film108 a, because stable electrical characteristics of a transistor can beachieved. However, when y₁ is three or more times as large as x₁, thefield-effect mobility of the transistor including the oxidesemiconductor film 108 a is reduced. Accordingly, y₁ is preferablysmaller than three times x₁.

In the case where the oxide semiconductor film 108 a is an In-M-Zn oxideand a target having the atomic ratio of metal elements ofIn:M:Zn=x₁:y₁:z₁ is used for depositing the oxide semiconductor film 108a, x₁/y₁ is preferably greater than or equal to ⅓ and less than or equalto 6, further preferably greater than or equal to 1 and less than orequal to 6, and z₁/y₁ is preferably greater than or equal to ⅓ and lessthan or equal to 6, further preferably greater than or equal to 1 andless than or equal to 6. Note that when z₁/y₁ is greater than or equalto 1 and less than or equal to 6, a CAAC-OS to be described later iseasily foliated as the oxide semiconductor film 108 a. Typical examplesof the atomic ratio of the metal elements of the target areIn:M:Zn=1:1:1 and In:M:Zn=3:1:2.

In the case where the oxide semiconductor films 108 b and 108 c are eachan In-M-Zn oxide and a target having an atomic ratio of metal elementsof In:M:Zn=x₂:y₂:z₂ is used for depositing the oxide semiconductor films108 b and 108 c, x₂/y₂ is preferably less than x₁/y₁, and z₂/y₂ ispreferably greater than or equal to ⅓ and less than or equal to 6,further preferably greater than or equal to 1 and less than or equal to6. When the atomic ratio of M with respect to indium is high, the energygap of the oxide semiconductor films 108 b and 108 c can be large andthe electron affinity thereof can be small; therefore, y₂/x₂ ispreferably higher than or equal to 3 or higher than or equal to 4.Typical examples of the atomic ratio of the metal elements of the targetinclude In:M:Zn=1:3:2, In:M:Zn=1:3:4, In:M:Zn=1:3:5, In:M:Zn=1:3:6,In:M:Zn=1:4:2, In:M:Zn=1:4:4, In:M:Zn=1:4:5, and In:M:Zn=1:5:5.

Furthermore, in the case where the oxide semiconductor films 108 b and108 c are each an In-M oxide, when a divalent metal element (e.g., zinc)is not included as M, the oxide semiconductor films 108 b and 108 cwhich do not include a spinel crystal structure can be formed. As theoxide semiconductor films 108 b and 108 c, for example, an In—Ga oxidefilm can be used. The In—Ga oxide can be formed by a sputtering methodusing an In—Ga metal oxide target (In:Ga=7:93), for example. To depositthe oxide semiconductor films 108 b and 108 c by a sputtering methodusing DC discharge, on the assumption that an atomic ratio of In:M isx:y, it is preferable that y/(x+y) be less than or equal to 0.96,further preferably less than or equal to 0.95, for example, 0.93.

In each of the oxide semiconductor films 108 a, 108 b, and 108 c, theproportions of the atoms in the above atomic ratio vary within a rangeof ±40% as an error.

The structures of the transistors of this embodiment can be freelycombined with each other.

<Method 1 for Manufacturing Semiconductor Device>

Next, a method for manufacturing the transistor 100 that is asemiconductor device of one embodiment of the present invention isdescribed below in detail with reference to FIGS. 10A to 10D and FIGS.11A to 11D.

Note that the films included in the transistor 100 (i.e., the insulatingfilm, the oxide semiconductor film, the conductive film, and the like)can be formed by any of a sputtering method, a chemical vapor deposition(CVD) method, a vacuum evaporation method, and a pulsed laser deposition(PLD) method. Alternatively, a coating method or a printing method canbe used. Although the sputtering method and a PECVD method are typicalexamples of the film formation method, a thermal CVD method may be used.As the thermal CVD method, an MOCVD method or an ALD method may be used,for example.

Deposition by the thermal CVD method may be performed in such a mannerthat the pressure in a chamber is set to an atmospheric pressure or areduced pressure, and a source gas and an oxidizer are supplied to thechamber at a time and react with each other in the vicinity of thesubstrate or over the substrate. Thus, no plasma is generated in thedeposition; therefore, the thermal CVD method has an advantage that nodefect due to plasma damage is caused.

Deposition by the ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching valves (also referred to as high-speed valves).In such a case, a first source gas is introduced, an inert gas (e.g.,argon or nitrogen) or the like is introduced at the same time as orafter introduction of the first gas so that the source gases are notmixed, and then a second source gas is introduced. Note that in the casewhere the first source gas and the inert gas are introduced at a time,the inert gas serves as a carrier gas, and the inert gas may also beintroduced at the same time as the introduction of the second sourcegas. Alternatively, the first source gas may be exhausted by vacuumevacuation instead of the introduction of the inert gas, and then thesecond source gas may be introduced. The first source gas is adsorbed onthe surface of the substrate to form a first single-atomic layer; thenthe second source gas is introduced to react with the firstsingle-atomic layer; as a result, a second single-atomic layer isstacked over the first single-atomic layer, so that a thin film isformed.

The sequence of the gas introduction is repeated plural times until adesired thickness is obtained, whereby a thin film with excellent stepcoverage can be formed. The thickness of the thin film can be adjustedby the number of repetition times of the sequence of the gasintroduction; therefore, an ALD method makes it possible to accuratelyadjust a thickness and thus is suitable for manufacturing a minutetransistor.

First, a conductive film is formed over the substrate 102 and processedthrough a lithography process and an etching process, whereby theconductive film 104 functioning as a gate electrode is formed. Then, theinsulating films 106 and 107 functioning as gate insulating films areformed over the conductive film 104 (see FIG. 10A).

The conductive film 104 functioning as a gate electrode can be formed bya sputtering method, a CVD method, a vacuum evaporation method, or a PLDmethod. Alternatively, a coating method or a printing method can beused. Although typical deposition methods are a sputtering method andPECVD method, a thermal CVD method, such as an MOCVD method, or an ALDmethod described above may be used.

In this embodiment, a glass substrate is used as the substrate 102, andas the conductive film 104 functioning as a gate electrode, a100-nm-thick tungsten film is formed by a sputtering method.

The insulating films 106 and 107 functioning as gate insulating filmscan be formed by a sputtering method, a PECVD method, a thermal CVDmethod, a vacuum evaporation method, a PLD method, or the like. In thisembodiment, a 400-nm-thick silicon nitride film as the insulating film106 and a 50-nm-thick silicon oxynitride film as the insulating film 107are formed by a PECVD method.

Note that the insulating film 106 can have a stacked-layer structure ofsilicon nitride films. Specifically, the insulating film 106 can have athree-layer stacked-layer structure of a first silicon nitride film, asecond silicon nitride film, and a third silicon nitride film. Anexample of the three-layer stacked-layer structure is as follows.

For example, the first silicon nitride film can be formed to have athickness of 50 nm under the condition where silane at a flow rate of200 sccm, nitrogen at a flow rate of 2000 sccm, and an ammonia gas at aflow rate of 100 sccm are supplied as a source gas to a reaction chamberof a PECVD apparatus; the pressure in the reaction chamber is controlledto 100 Pa, and a power of 2000 W is supplied using a 27.12 MHzhigh-frequency power source.

The second silicon nitride film can be formed to have a thickness of 300nm under the condition where silane at a flow rate of 200 sccm, nitrogenat a flow rate of 2000 sccm, and an ammonia gas at a flow rate of 2000sccm are supplied as a source gas to the reaction chamber of the PECVDapparatus; the pressure in the reaction chamber is controlled to 100 Pa,and a power of 2000 W is supplied using a 27.12 MHz high-frequency powersource.

The third silicon nitride film can be formed to have a thickness of 50nm under the condition where silane at a flow rate of 200 sccm andnitrogen at a flow rate of 5000 sccm are supplied as a source gas to thereaction chamber of the PECVD apparatus; the pressure in the reactionchamber is controlled to 100 Pa, and a power of 2000 W is supplied usinga 27.12 MHz high-frequency power source.

Note that the first silicon nitride film, the second silicon nitridefilm, and the third silicon nitride film can be each formed at asubstrate temperature of 350° C.

When the insulating film 106 has the three-layer stacked-layer structureof silicon nitride films, for example, in the case where a conductivefilm including Cu is used as the conductive film 104, the followingeffect can be obtained.

The first silicon nitride film can inhibit diffusion of a copper (Cu)element from the conductive film 104. The second silicon nitride filmhas a function of releasing hydrogen and can improve withstand voltageof the insulating film functioning as a gate insulating film. The thirdsilicon nitride film releases a small amount of hydrogen and can inhibitdiffusion of hydrogen released from the second silicon nitride film.

The insulating film 107 is preferably an insulating film includingoxygen to improve characteristics of an interface with the oxidesemiconductor film 108 formed later.

Next, the oxide semiconductor film 108 is formed over the insulatingfilm 107 (see FIG. 10B).

In this embodiment, an oxide semiconductor film is formed by asputtering method using an In—Ga—Zn metal oxide target (having an atomicratio of In:Ga:Zn=1:1:1.2), a mask is formed over the oxidesemiconductor film through a lithography process, and the oxidesemiconductor film is processed into a desired region, whereby the oxidesemiconductor film 108 having an island shape is formed.

After the oxide semiconductor film 108 is formed, heat treatment may beperformed at a temperature higher than or equal to 150° C. and lowerthan the strain point of the substrate, preferably higher than or equalto 200° C. and lower than or equal to 450° C., further preferably higherthan or equal to 300° C. and lower than or equal to 450° C. The heattreatment performed here serves as one kind of treatment for increasingthe purity of the oxide semiconductor film and can reduce hydrogen,water, and the like included in the oxide semiconductor film 108. Notethat the heat treatment for the purpose of reducing hydrogen, water, andthe like may be performed before the oxide semiconductor film 108 isprocessed into an island shape.

An electric furnace, an RTA apparatus, or the like can be used for theheat treatment performed on the oxide semiconductor film 108. With theuse of an RTA apparatus, the heat treatment can be performed at atemperature higher than or equal to the strain point of the substrate ifthe heating time is short. Therefore, the heat treatment time can beshortened.

Note that the heat treatment performed on the oxide semiconductor film108 may be performed under an atmosphere of nitrogen, oxygen, ultra-dryair (air in which a water content is 20 ppm or less, preferably 1 ppm orless, further preferably 10 ppb or less), or a rare gas (argon, helium,or the like). The atmosphere of nitrogen, oxygen, ultra-dry air, or arare gas preferably does not contain hydrogen, water, and the like.Furthermore, after heat treatment performed in a nitrogen atmosphere ora rare gas atmosphere, heat treatment may be additionally performed inan oxygen atmosphere or an ultra-dry air atmosphere. As a result,hydrogen, water, and the like can be released from the oxidesemiconductor film and oxygen can be supplied to the oxide semiconductorfilm at the same time. Consequently, the amount of oxygen vacancies inthe oxide semiconductor film can be reduced.

In the case where the oxide semiconductor film 108 is formed by asputtering method, as a sputtering gas, a rare gas (typically argon),oxygen, or a mixed gas of a rare gas and oxygen is used as appropriate.In the case of using the mixed gas of a rare gas and oxygen, theproportion of oxygen to a rare gas is preferably increased. In addition,increasing the purity of a sputtering gas is necessary. For example, asan oxygen gas or an argon gas used for a sputtering gas, a gas which ishighly purified to have a dew point of −40° C. or lower, preferably −80°C. or lower, further preferably −100° C. or lower, still furtherpreferably −120° C. or lower is used, whereby entry of moisture and thelike into the oxide semiconductor film 108 can be minimized.

In the case where the oxide semiconductor film 108 is formed by asputtering method, a chamber in a sputtering apparatus is preferablyevacuated to be a high vacuum state (to the degree of about 5×10⁻⁷ Pa to1×10⁻⁴ Pa) with an adsorption vacuum evacuation pump such as a cryopumpin order to remove water or the like, which serves as an impurity forthe oxide semiconductor film 108, as much as possible. Alternatively, aturbo molecular pump and a cold trap are preferably combined so as toprevent a backflow of a gas, especially a gas including carbon orhydrogen from an exhaust system to the inside of the chamber.

Next, the conductive films 112 a and 112 b functioning as a sourceelectrode and a drain electrode are formed over the insulating film 107and the oxide semiconductor film 108 (see FIG. 10C).

In this embodiment, the conductive films 112 a and 112 b are formed inthe following manner: a stack formed of a 50-nm-thick tungsten film anda 400-nm-thick aluminum film is formed by a sputtering method, a mask isformed over the stack through a lithography process, and the stack isprocessed into desired regions. Although the conductive films 112 a and112 b each have a two-layer stacked structure in this embodiment, oneembodiment of the present invention is not limited thereto. For example,the conductive films 112 a and 112 b each may have a three-layerstacked-layer structure formed of a 50-nm-thick tungsten film, a400-nm-thick aluminum film, and a 100-nm-thick titanium film.

After the conductive films 112 a and 112 b are formed, a surface of theoxide semiconductor film 108 (on a back channel side) may be cleaned.The cleaning may be performed, for example, using a chemical solutionsuch as phosphoric acid. The cleaning using a chemical solution such asa phosphoric acid can remove impurities (e.g., an element included inthe conductive films 112 a and 112 b and the like) attached to thesurface of the oxide semiconductor film 108.

Note that a recessed portion might be formed in part of the oxidesemiconductor film 108 at the step of forming the conductive films 112 aand 112 b and/or the cleaning step.

Through the steps, the transistor 100 is formed.

Next, over the transistor 100, specifically, over the oxidesemiconductor film 108 and the conductive films 112 a and 112 b of thetransistor 100, the insulating films 114 and 116 functioning asprotective insulating films of the transistor 100 are formed (see FIG.10D).

Note that after the insulating film 114 is formed, the insulating film116 is preferably formed in succession without exposure to the air.After the insulating film 114 is formed, the insulating film 116 isformed in succession by adjusting at least one of the flow rate of asource gas, pressure, a high-frequency power, and a substratetemperature without exposure to the air, whereby the concentration ofimpurities attributed to the atmospheric component at the interfacebetween the insulating film 114 and the insulating film 116 can bereduced and oxygen in the insulating films 114 and 116 can be moved tothe oxide semiconductor film 108; accordingly, the amount of oxygenvacancy in the oxide semiconductor film 108 can be reduced.

For example, as the insulating film 114, a silicon oxynitride film canbe formed by a PECVD method. In this case, a deposition gas includingsilicon and an oxidizing gas are preferably used as a source gas.Typical examples of the deposition gas including silicon include silane,disilane, trisilane, and silane fluoride. Examples of the oxidizing gasinclude dinitrogen monoxide and nitrogen dioxide. An insulating filmincluding nitrogen and having a small number of defects can be formed asthe insulating film 114 by a PECVD method under the conditions where theratio of the oxidizing gas to the deposition gas is higher than 20 timesand lower than 100 times, preferably higher than or equal to 40 timesand lower than or equal to 80 times and the pressure in a treatmentchamber is lower than 100 Pa, preferably lower than or equal to 50 Pa.

In this embodiment, a silicon oxynitride film is formed as theinsulating film 114 by a PECVD method under the conditions where thesubstrate 102 is held at a temperature of 220° C., silane at a flow rateof 50 sccm and dinitrogen monoxide at a flow rate of 2000 sccm are usedas a source gas, the pressure in the treatment chamber is 20 Pa, and ahigh-frequency power of 100 W at 13.56 MHz (1.6×10⁻² W/cm² as the powerdensity) is supplied to parallel-plate electrodes.

As the insulating film 116, a silicon oxide film or a silicon oxynitridefilm is formed under the conditions where the substrate placed in atreatment chamber of the PECVD apparatus that is vacuum-evacuated isheld at a temperature higher than or equal to 180° C. and lower than orequal to 280° C., preferably higher than or equal to 200° C. and lowerthan or equal to 240° C., the pressure is greater than or equal to 100Pa and less than or equal to 250 Pa, preferably greater than or equal to100 Pa and less than or equal to 200 Pa with introduction of a sourcegas into the treatment chamber, and a high-frequency power of greaterthan or equal to 0.17 W/cm² and less than or equal to 0.5 W/cm²,preferably greater than or equal to 0.25 W/cm² and less than or equal to0.35 W/cm² is supplied to an electrode provided in the treatmentchamber.

As the deposition conditions of the insulating film 116, thehigh-frequency power having the above power density is supplied to areaction chamber having the above pressure, whereby the degradationefficiency of the source gas in plasma is increased, oxygen radicals areincreased, and oxidation of the source gas is promoted; thus, the oxygencontent in the insulating film 116 becomes higher than that in thestoichiometric composition. On the other hand, in the film formed at asubstrate temperature within the above temperature range, the bondbetween silicon and oxygen is weak, and accordingly, part of oxygen inthe film is released by heat treatment in a later step. Thus, it ispossible to form an oxide insulating film which includes oxygen inexcess of that in the stoichiometric composition and from which part ofoxygen is released by heating.

Note that the insulating film 114 functions as a protective film for theoxide semiconductor film 108 in the step of forming the insulating film116. Therefore, the insulating film 116 can be formed using thehigh-frequency power having a high power density while damage to theoxide semiconductor film 108 is reduced.

Note that in the deposition conditions of the insulating film 116, whenthe flow rate of the deposition gas including silicon with respect tothe oxidizing gas is increased, the number of defects in the insulatingfilm 116 can be reduced. Typically, it is possible to form an oxideinsulating layer in which the number of defects is small, i.e., the spindensity of a signal which appears at g=2.001 originating from a danglingbond of silicon is lower than 6×10¹⁷ spins/cm³, preferably lower than orequal to 3×10¹⁷ spins/cm³, further preferably lower than or equal to1.5×10¹⁷ spins/cm³ by ESR measurement. As a result, the reliability ofthe transistor can be improved.

Heat treatment may be performed after the insulating films 114 and 116are formed. The heat treatment can reduce nitrogen oxide included in theinsulating films 114 and 116. By the heat treatment, part of oxygenincluded in the insulating films 114 and 116 can be moved to the oxidesemiconductor film 108, so that the amount of oxygen vacancy included inthe oxide semiconductor film 108 can be reduced.

The temperature of the heat treatment performed on the insulating films114 and 116 is typically higher than or equal to 150° C. and lower thanor equal to 400° C., preferably higher than or equal to 300° C. andlower than or equal to 400° C., further preferably higher than or equalto 320° C. and lower than or equal to 370° C. The heat treatment may beperformed under an atmosphere of nitrogen, oxygen, ultra-dry air (air inwhich a water content is 20 ppm or less, preferably 1 ppm or less,further preferably 10 ppb or less), or a rare gas (argon, helium, andthe like). Note that an electric furnace, an RTA apparatus, and the likecan be used for the heat treatment, in which it is preferable thathydrogen, water, and the like not be included in the nitrogen, oxygen,ultra-dry air, or rare gas.

In this embodiment, the heat treatment is performed at 350° C. for 1hour in an atmosphere of nitrogen and oxygen.

Next, a film 130 that inhibits release of oxygen is formed over theinsulating film 116 (see FIG. 11A).

The film 130 that inhibits release of oxygen includes at least one ofindium, zinc, titanium, aluminum, tungsten, tantalum, and molybdenum.For example, a conductive material such as an alloy including any of themetal elements, an alloy including any of the metal elements incombination, a metal oxide including any of the metal elements, a metalnitride including any of the metal elements, or a metal nitride oxideincluding any of the metal elements is used.

The film 130 that inhibits release of oxygen can be formed using, forexample, a tantalum nitride film, a titanium film, an indium tin oxide(ITO) film, an aluminum film, or an oxide semiconductor film (e.g., anIGZO film having an atomic ratio of In:Ga:Zn=1:4:5).

Next, oxygen 141 is added to the insulating films 114 and 116 and theoxide semiconductor film 108 through the film 130 (see FIG. 11B).

The thickness of the film 130 that inhibits release of oxygen can begreater than or equal to 1 nm and less than or equal to 20 nm, orgreater than or equal to 2 nm and less than or equal to 10 nm. In thisembodiment, a 5-nm-thick tantalum nitride film is used as the film 130.

As a method for adding the oxygen 141 to the insulating films 114 and116 and the oxide semiconductor film 108 through the film 130, an iondoping method, an ion implantation method, plasma treatment, or the likeis given. When the film 130 is provided over the insulating film 116 andthen oxygen is added, the film 130 serves as a protective film forpreventing oxygen from being released from the insulating film 116.Thus, a larger amount of oxygen can be added to the insulating films 114and 116 and the oxide semiconductor film 108.

In the case where oxygen is introduced by plasma treatment, by makingoxygen excited by a microwave to generate high density oxygen plasma,the amount of oxygen introduced into the insulating film 116 can beincreased.

Note that by the addition of the oxygen 141, the film 130 becomes theinsulating film 131 formed of oxide or nitride of metal (indium, zinc,titanium, aluminum, tungsten, tantalum, or molybdenum) (see FIG. 11C).

Note that the insulating film 131 might be a conductor or asemiconductor in the case where the treatment for adding the oxygen 141is insufficiently performed, or even in the case where the treatment issufficiently performed depending on the metal material used for the film130. Note that since the insulating film 131 is positioned on the backchannel side of the transistor 100, when the insulating film 131 is aconductor or a semiconductor, an electron serving as a carrier might betrapped in the insulating film 131; therefore, the insulating film 131is preferably an insulator.

After that, the insulating film 131 is removed, and the insulating film118 is formed over the insulating film 116 (see FIG. 11D).

Note that heat treatment may be performed before or after the formationof the insulating film 118, so that excess oxygen included in theinsulating films 114 and 116 can diffuse into the oxide semiconductorfilm 108 to fill oxygen vacancy in the oxide semiconductor film 108.Alternatively, the insulating film 118 may be deposited by heating, sothat excess oxygen included in the insulating films 114 and 116 candiffuse into the oxide semiconductor film 108 to fill oxygen vacancy inthe oxide semiconductor film 108.

In the case where the insulating film 118 is formed by a PECVD method,the substrate temperature is preferably set to higher than or equal to300° C. and lower than or equal to 400° C., more preferably higher thanor equal to 320° C. and lower than or equal to 370° C., so that a densefilm can be formed.

For example, in the case where a silicon nitride film is formed by aPECVD method as the insulating film 118, a deposition gas containingsilicon, nitrogen, and ammonia are preferably used as a source gas. Asmall amount of ammonia compared to the amount of nitrogen is used,whereby ammonia is dissociated in the plasma and activated species aregenerated. The activated species cleave a bond between silicon andhydrogen which are included in a deposition gas containing silicon and atriple bond between nitrogen molecules. As a result, a dense siliconnitride film having few defects, in which bonds between silicon andnitrogen are promoted and bonds between silicon and hydrogen is few, canbe formed. On the other hand, when the amount of ammonia with respect tonitrogen is large, decomposition of a deposition gas containing siliconand decomposition of nitrogen are not promoted, so that a sparse siliconnitride film in which bonds between silicon and hydrogen remain anddefects are increased is formed. Therefore, in the source gas, a flowrate ratio of the nitrogen to the ammonia is set to be greater than orequal to 5 and less than or equal to 50, preferably greater than orequal to 10 and less than or equal to 50.

In this embodiment, with the use of a PECVD apparatus, a 50-nm-thicksilicon nitride film is formed as the insulating film 118 using silane,nitrogen, and ammonia as a source gas. The flow rate of silane is 50sccm, the flow rate of nitrogen is 5000 sccm, and the flow rate ofammonia is 100 sccm. The pressure in the treatment chamber is 100 Pa,the substrate temperature is 350° C., and high-frequency power of 1000 Wis supplied to parallel-plate electrodes with a 27.12 MHz high-frequencypower source. Note that the PECVD apparatus is a parallel-plate PECVDapparatus in which the electrode area is 6000 cm², and the power perunit area (power density) into which the supplied power is converted is1.7×10⁻¹ W/cm².

Heat treatment may be performed after the formation of the insulatingfilm 118. The heat treatment is performed typically at a temperature ofhigher than or equal to 150° C. and lower than or equal to 400° C.,preferably higher than or equal to 300° C. and lower than or equal to400° C., further preferably higher than or equal to 320° C. and lowerthan or equal to 370° C. When the heat treatment is performed, theamount of hydrogen and water in the insulating films 114 and 116 isreduced and accordingly the generation of defects in the oxidesemiconductor film 108 described above is inhibited.

Through the above process, the semiconductor device illustrated in FIGS.1A to 1C can be manufactured.

Note that the transistor 100A in FIGS. 2A and 2B can be manufactured byforming the insulating film 118 without removal of the insulating film131.

<Method 2 for Manufacturing Semiconductor Device>

Next, a method for manufacturing the transistor 150 in FIGS. 3A to 3Cthat is a semiconductor device of one embodiment of the presentinvention is described below in detail with reference to FIGS. 12A to12D and FIGS. 13A and 13B.

First, the steps up to the step in FIG. 10B are performed, and then theinsulating films 114 and 116 and the film 130 that inhibits release ofoxygen are formed over the oxide semiconductor film 108 (see FIG. 12A).

Next, the oxygen 141 is added to the insulating films 114 and 116 andthe oxide semiconductor film 108 through the film 130 (see FIG. 12B).

Note that by the addition of the oxygen 141, the film 130 becomes theinsulating film 131 formed of oxide or nitride of metal (indium, zinc,titanium, aluminum, tungsten, tantalum, or molybdenum) (see FIG. 12C).

After that, the insulating film 131 is removed, a mask is formed overthe insulating film 116 through a lithography process, and the openings141 a and 141 b are formed in desired regions in the insulating films114 and 116. Note that the openings 141 a and 141 b reach the oxidesemiconductor film 108 (see FIG. 12D).

Next, a conductive film is deposited over the oxide semiconductor film108 and the insulating film 116 to cover the openings 141 a and 141 b, amask is formed over the conductive film through a lithography process,and the conductive film is processed into desired regions, whereby theconductive films 112 a and 112 b are formed (see FIG. 13A).

Next, the insulating film 118 is formed over the insulating film 116 andthe conductive films 112 a and 112 b (see FIG. 13B).

Through the above process, the semiconductor device illustrated in FIGS.3A to 3C can be manufactured.

Note that the transistor 150A in FIGS. 4A and 4B can be manufactured byforming the insulating film 118 without removal of the insulating film131.

<Method 3 for Manufacturing Semiconductor Device>

Next, a method for manufacturing the transistor 170 that is asemiconductor device of one embodiment of the present invention isdescribed below in detail with reference to FIGS. 14A to 14D and FIGS.15A to 15D.

FIGS. 14A and 14C and FIGS. 15A and 15C are each a cross-sectional viewin the channel length direction of the transistor 170 and FIGS. 14B and14D and FIGS. 15B and 15D are each a cross-sectional view in the channelwidth direction of the transistor 170.

First, the steps up to the step in FIG. 11D are performed (see FIGS. 14Aand 14B).

Next, a mask is formed over the insulating film 118 through alithography process, and the opening 142 c is formed in a desired regionin the insulating films 114, 116, and 118. In addition, a mask is formedover the insulating film 118 through a lithography process, and theopenings 142 a and 142 b are formed in desired regions in the insulatingfilms 106, 107, 114, 116, and 118. Note that the opening 142 c reachesthe conductive film 112 b. The openings 142 a and 142 b reach theconductive film 104 (see FIGS. 14C and 14D).

Note that the openings 142 a and 142 b and the opening 140 c may beformed at a time or may be formed by different steps. In the case wherethe openings 142 a and 142 b and the opening 140 c are formed at a time,for example, a gray-tone mask or a half-tone mask may be used.

Next, a conductive film 120 is formed over the insulating film 118 tocover the openings 142 a, 142 b, and 142 c (see FIGS. 15A and 15B).

For the conductive film 120, for example, a material including one ofindium (In), zinc (Zn), and tin (Sn) can be used. In particular, for theconductive film 120, a light-transmitting conductive material such asindium oxide including tungsten oxide, indium zinc oxide includingtungsten oxide, indium oxide including titanium oxide, indium tin oxideincluding titanium oxide, indium tin oxide (ITO), indium zinc oxide, orindium tin oxide to which silicon oxide is added (ITSO) can be used. Theconductive film 120 can be formed by a sputtering method, for example.In this embodiment, a 110-nm-thick ITSO film is formed by a sputteringmethod.

Next, a mask is formed over the conductive film 120 through alithography process, and the conductive film 120 is processed intodesired regions to form the conductive films 120 a and 120 b (see FIGS.15C and 15D).

Through the above process, the transistor 170 illustrated in FIGS. 7Aand 7B can be manufactured.

<Method 4 for Manufacturing Semiconductor Device>

Next, a method for manufacturing the transistor 100 that is asemiconductor device of one embodiment of the present invention, whichis different from that described in <Method 1 for manufacturingsemiconductor device>, is described below with reference to FIGS. 16A to16D.

First, the steps up to the step in FIG. 10C are performed to form thetransistor 100. After that, over the transistor 100, specifically, overthe oxide semiconductor film 108 and the conductive films 112 a and 112b, the insulating film 114 is formed. Then, the film 130 that inhibitsrelease of oxygen is formed over the insulating film 114 (see FIG. 16A).

Next, the oxygen 141 is added to the insulating film 114 and the oxidesemiconductor film 108 through the film 130 (see FIG. 16B).

Note that by the addition of the oxygen 141, the film 130 becomes theinsulating film 131 formed of oxide or nitride of metal (indium, zinc,titanium, aluminum, tungsten, tantalum, or molybdenum) (see FIG. 16C).

Next, the insulating film 131 is removed, and the insulating film 116 isformed over the insulating film 114. After that, the insulating film 118is formed over the insulating film 116 (see FIG. 16D).

Through the above process, the semiconductor device illustrated in FIGS.1A to 1C can be manufactured.

Note that in a process for manufacturing the transistor 100B in FIGS. 2Cand 2D, the insulating film 116 and the insulating film 118 can beformed without removal of the insulating film 131.

<Method 5 for Manufacturing Semiconductor Device>

The above-described semiconductor device of one embodiment of thepresent invention may be formed in combination with a manufacturingmethod illustrated in FIGS. 17A to 17D, as appropriate.

First, an insulating film 101 is formed over the substrate 102, and thefilm 130 that inhibits release of oxygen is formed over the insulatingfilm 101 (see FIG. 17A).

A material which can be used for the insulating film 107 can be used forthe insulating film 101.

Next, the oxygen 141 is added to the insulating film 101 through thefilm 130 (see FIG. 17B).

Note that by the addition of the oxygen 141, the film 130 becomes theinsulating film 131 formed of oxide or nitride of metal (indium, zinc,titanium, aluminum, tungsten, tantalum, or molybdenum) (see FIG. 17C).

Next, the insulating film 131 is removed, and the conductive film 104 isformed over the insulating film 101. Then, the insulating films 106 and107 are formed over the insulating film 101 and the conductive film 104(see FIG. 17D).

In this manner, the transistor that is the semiconductor device of oneembodiment of the present invention may include the base film. Inaddition, a region including excess oxygen may be formed in the basefilm by oxygen addition treatment. With such a structure, oxygen in thebase film can diffuse into the oxide semiconductor film 108 through theinsulating films 106 and 107 to fill oxygen vacancy in the oxidesemiconductor film 108.

<Method 6 for Manufacturing Semiconductor Device>

The above-described semiconductor device of one embodiment of thepresent invention may be formed in combination with a manufacturingmethod illustrated in FIGS. 18A to 18C, as appropriate.

First, the steps up to the step in FIG. 10A are performed, and the film130 that inhibits release of oxygen is formed over the insulating film107 (see FIG. 18A).

Next, the oxygen 141 is added to the insulating film 107 through thefilm 130 (see FIG. 18B).

Note that by the addition of the oxygen 141, the film 130 becomes theinsulating film 131 formed of oxide or nitride of metal (indium, zinc,titanium, aluminum, tungsten, tantalum, or molybdenum) (see FIG. 18C).

After that, the insulating film 131 is removed, the oxide semiconductorfilm 108 is formed over the insulating film 107, and steps after thestep of FIG. 10B are performed.

As described above, oxygen addition treatment may be performed on theinsulating film 107 serving as part of the gate insulating film toincrease the oxygen content of the insulating film 107 in the processfor manufacturing the transistor that is the semiconductor device of oneembodiment of the present invention.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 2

In this embodiment, the structure of an oxide semiconductor included ina semiconductor device of one embodiment of the present invention isdescribed below in detail.

First a structure which can be included in an oxide semiconductor isdescribed below.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a c-axis alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor. Inaddition, examples of a crystalline oxide semiconductor include a singlecrystal oxide semiconductor, a CAAC-OS, a polycrystalline oxidesemiconductor, and an nc-OS.

It is known that an amorphous structure is generally defined as beingmetastable and unfixed, and being isotropic and having no non-uniformstructure. In other words, an amorphous structure has a flexible bondangle and a short-range order but does not have a long-range order.

This means that an inherently stable oxide semiconductor cannot beregarded as a completely amorphous oxide semiconductor. Moreover, anoxide semiconductor that is not isotropic (e.g., an oxide semiconductorfilm that has a periodic structure in a microscopic region) cannot beregarded as a completely amorphous oxide semiconductor. Note that ana-like OS has a periodic structure in a microscopic region, but at thesame time has a void and has an unstable structure. For this reason, ana-like OS has physical properties similar to those of an amorphous oxidesemiconductor.

<CAAC-OS>

First, a CAAC-OS is described.

A CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, in the high-resolutionTEM image, a boundary between pellets, that is, a grain boundary is notclearly observed. Thus, in the CAAC-OS, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

The CAAC-OS observed with a TEM is described below. FIG. 19A shows ahigh-resolution TEM image of a cross section of the CAAC-OS which isobserved from a direction substantially parallel to the sample surface.The high-resolution TEM image is obtained with a spherical aberrationcorrector function. The high-resolution TEM image obtained with aspherical aberration corrector function is particularly referred to as aCs-corrected high-resolution TEM image. The Cs-corrected high-resolutionTEM image can be obtained with, for example, an atomic resolutionanalytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 19B is an enlarged Cs-corrected high-resolution TEM image of aregion (1) in FIG. 19A. FIG. 19B shows that metal atoms are arranged ina layered manner in a pellet. Each metal atom layer has a configurationreflecting unevenness of a surface over which a CAAC-OS film is formed(hereinafter, the surface is referred to as a formation surface) or atop surface of the CAAC-OS, and is arranged parallel to the formationsurface or the top surface of the CAAC-OS.

As shown in FIG. 19B, the CAAC-OS has a characteristic atomicarrangement. The characteristic atomic arrangement is denoted by anauxiliary line in FIG. 19C. FIGS. 19B and 19C prove that the size of apellet is approximately 1 nm to 3 nm, and the size of a space caused bytilt of the pellets is approximately 0.8 nm. Therefore, the pellet canalso be referred to as a nanocrystal (nc). Furthermore, a CAAC-OS can bereferred to as an oxide semiconductor including c-axis alignednanocrystals (CANC).

Here, according to the Cs-corrected high-resolution TEM images, theschematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120is illustrated by such a structure in which bricks or blocks are stacked(see FIG. 19D). The part in which the pellets are tilted as observed inFIG. 19C corresponds to a region 5161 shown in FIG. 19D.

FIG. 20A shows a Cs-corrected high-resolution TEM image of a plane ofthe CAAC-OS observed from a direction substantially perpendicular to thesample surface. FIGS. 20B, 20C, and 20D are enlarged Cs-correctedhigh-resolution TEM images of regions (1), (2), and (3) in FIG. 20A,respectively. FIGS. 20B, 20C, and 20D indicate that metal atoms arearranged in a triangular, quadrangular, or hexagonal configuration in apellet. However, there is no regularity of arrangement of metal atomsbetween different pellets.

Next, a CAAC-OS analyzed by X-ray diffraction (XRD) is described. Forexample, when the structure of a CAAC-OS including an InGaZnO₄ crystalis analyzed by an out-of-plane method, a peak appears at a diffractionangle (2θ) of around 31° as shown in FIG. 21A. This peak is derived fromthe (009) plane of the InGaZnO₄ crystal, which indicates that crystalsin the CAAC-OS have c-axis alignment, and that the c-axes are aligned ina direction substantially perpendicular to the formation surface or thetop surface of the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-planemethod, another peak may appear when 2θ is around 36°, in addition tothe peak at 2θ of around 31°. The peak of 2θ at around 36° indicatesthat a crystal having no c-axis alignment is included in part of theCAAC-OS. It is preferable that in the CAAC-OS analyzed by anout-of-plane method, a peak appear when 2θ is around 31° and that a peaknot appear when 2θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray is incident on a sample in a directionsubstantially perpendicular to the c-axis, a peak appears when 2θ isaround 56°. This peak is derived from the (110) plane of the InGaZnO₄crystal. In the case of the CAAC-OS, when analysis (ϕ scan) is performedwith 2q fixed at around 56° and with the sample rotated using a normalvector of the sample surface as an axis (ϕ axis), as shown in FIG. 21B,a peak is not clearly observed. In contrast, in the case of a singlecrystal oxide semiconductor of InGaZnO₄, when f scan is performed with2θ fixed at around 56°, as shown in FIG. 21C, six peaks which arederived from crystal planes equivalent to the (110) plane are observed.Accordingly, the structural analysis using XRD shows that the directionsof a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the sample surface, a diffraction pattern (also referred toas a selected-area transmission electron diffraction pattern) shown inFIG. 50A can be obtained. In this diffraction pattern, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are included. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OShave c-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 50B shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 50B, a ring-like diffraction pattern isobserved. Thus, the electron diffraction also indicates that the a-axesand b-axes of the pellets included in the CAAC-OS do not have regularalignment. The first ring in FIG. 50B is considered to be derived fromthe (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal.Furthermore, it is supposed that the second ring in FIG. 50B is derivedfrom the (110) plane and the like.

As described above, the CAAC-OS is an oxide semiconductor with highcrystallinity. Entry of impurities, formation of defects, or the likemight decrease the crystallinity of an oxide semiconductor. This meansthat the CAAC-OS has small amounts of impurities and defects (e.g.,oxygen vacancy).

Note that the impurity means an element other than the main componentsof the oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically, siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

The characteristics of an oxide semiconductor having impurities ordefects might be changed by light, heat, or the like. Impuritiesincluded in the oxide semiconductor might serve as carrier traps orcarrier generation sources, for example. Furthermore, oxygen vacancy inthe oxide semiconductor serves as a carrier trap or serves as a carriergeneration source when hydrogen is captured therein.

The CAAC-OS having small amounts of impurities and oxygen vacancy is anoxide semiconductor film with low carrier density (specifically, lowerthan 8×10¹¹/cm³, preferably lower than 1×10¹¹/cm³, further preferablylower than 1×10¹⁰/cm³, and is higher than or equal to 1×10⁻⁹/cm³). Suchan oxide semiconductor is referred to as a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. A CAAC-OShas a low impurity concentration and a low density of defect states.Thus, the CAAC-OS can be referred to as an oxide semiconductor havingstable characteristics.

<nc-OS>

Next, an nc-OS is described.

An nc-OS has a region in which a crystal part is observed and a regionin which a crystal part is not clearly observed in a high-resolution TEMimage. In most cases, the size of a crystal part included in the nc-OSis greater than or equal to 1 nm and less than or equal to 10 nm, orgreater than or equal to 1 nm and less than or equal to 3 nm. Note thatan oxide semiconductor including a crystal part whose size is greaterthan 10 nm and less than or equal to 100 nm is sometimes referred to asa microcrystalline oxide semiconductor. In a high-resolution TEM imageof the nc-OS, for example, a grain boundary is not clearly observed insome cases. Note that there is a possibility that the origin of thenanocrystal is the same as that of a pellet in a CAAC-OS. Therefore, acrystal part of the nc-OS may be referred to as a pellet in thefollowing description.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different pellets in thenc-OS. Thus, the orientation of the whole film is not observed.Accordingly, the nc-OS cannot be distinguished from an a-like OS and anamorphous oxide semiconductor, depending on an analysis method. Forexample, when the nc-OS is analyzed by an out-of-plane method using anX-ray beam having a diameter larger than the size of a pellet, a peakwhich shows a crystal plane does not appear. Furthermore, a diffractionpattern like a halo pattern is observed when the nc-OS is subjected toelectron diffraction using an electron beam with a probe diameter (e.g.,50 nm or larger) that is larger than the size of a pellet. Meanwhile,spots appear in a nanobeam electron diffraction pattern of the nc-OSwhen an electron beam having a probe diameter close to or smaller thanthe size of a pellet is applied. Moreover, in a nanobeam electrondiffraction pattern of the nc-OS, regions with high luminance in acircular (ring) pattern are shown in some cases. Also in a nanobeamelectron diffraction pattern of the nc-OS layer, a plurality of spots isshown in a ring-like region in some cases.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including random aligned nanocrystals (RANC) oran oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as comparedto an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an a-like OS and an amorphousoxide semiconductor. Note that there is no regularity of crystalorientation between different pellets in the nc-OS. Therefore, the nc-OShas a higher density of defect states than the CAAC-OS.

<a-Like OS>

An a-like OS has a structure intermediate between those of the nc-OS andthe amorphous oxide semiconductor.

In a high-resolution TEM image of the a-like OS, a void may be observed.Furthermore, in the high-resolution TEM image, there are a region wherea crystal part is clearly observed and a region where a crystal part isnot observed.

The a-like OS has an unstable structure because it includes a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS (sample A), an nc-OS (sample B), and a CAAC-OS (sample C)are prepared as samples subjected to electron irradiation. Each of thesamples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

Note that which part is regarded as a crystal part is determined asfollows. It is known that a unit cell of the InGaZnO₄ crystal has astructure in which nine layers including three In—O layers and sixGa—Zn—O layers are stacked in the c-axis direction. The distance betweenthe adjacent layers is equivalent to the lattice spacing on the (009)plane (also referred to as d value). The value is calculated to be 0.29nm from crystal structural analysis. Accordingly, a portion where thelattice spacing between lattice fringes is greater than or equal to 0.28nm and less than or equal to 0.30 nm is regarded as a crystal part ofInGaZnO₄. Each of lattice fringes corresponds to the a-b plane of theInGaZnO₄ crystal.

FIG. 51 shows change in the average size of crystal parts (at 22 pointsto 45 points) in each sample. Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 51 indicates thatthe crystal part size in the a-like OS increases with an increase in thecumulative electron dose. Specifically, as shown by (1) in FIG. 51, acrystal part of approximately 1.2 nm at the start of TEM observation(the crystal part is also referred to as an initial nucleus) grows to asize of approximately 2.6 nm at a cumulative electron dose of 4.2×10⁸e⁻/nm². In contrast, the crystal part size in the nc-OS and the CAAC-OSshows little change from the start of electron irradiation to acumulative electron dose of 4.2×10⁸ e⁻/nm². Specifically, as shown by(2) and (3) in FIG. 51, the average crystal sizes in an nc-OS and aCAAC-OS are approximately 1.4 nm and approximately 2.1 nm, respectively,regardless of the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS is inducedby electron irradiation. In contrast, in the nc-OS and the CAAC-OS,growth of the crystal part is hardly induced by electron irradiation.Therefore, the a-like OS has an unstable structure as compared with thenc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit includes a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of the single crystal oxide semiconductorhaving the same composition. Note that it is difficult to deposit anoxide semiconductor having a density of lower than 78% of the density ofthe single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomicratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that there is a possibility that an oxide semiconductor having acertain composition cannot exist in a single crystal structure. In thatcase, single crystal oxide semiconductors with different compositionsare combined at an adequate ratio, which makes it possible to calculatedensity equivalent to that of a single crystal oxide semiconductor withthe desired composition. The density of a single crystal oxidesemiconductor having the desired composition can be calculated using aweighted average according to the combination ratio of the singlecrystal oxide semiconductors with different compositions. Note that itis preferable to use as few kinds of single crystal oxide semiconductorsas possible to calculate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedlayer including two or more of an amorphous oxide semiconductor, ana-like OS, an nc-OS, and a CAAC-OS, for example.

<Deposition Model>

Deposition models of a CAAC-OS film and an nc-OS film are describedbelow.

FIG. 40A is a schematic diagram of a deposition chamber illustrating astate where the CAAC-OS film is formed by a sputtering method.

A target 1130 is attached to a backing plate. Under the target 1130 andthe backing plate, a plurality of magnets are provided. The plurality ofmagnets cause a magnetic field over the target 1130. A sputtering methodin which the disposition speed is increased by utilizing a magneticfield of magnets is referred to as a magnetron sputtering method.

The target 1130 has a polycrystalline structure in which a cleavageplane exists in at least one crystal grain. Note that the details of thecleavage plane are described later.

A substrate 1120 is placed to face the target 1130, and the distance d(also referred to as a target-substrate distance (T-S distance)) isgreater than or equal to 0.01 m and less than or equal to 1 m,preferably greater than or equal to 0.02 m and less than or equal to 0.5m. The deposition chamber is mostly filled with a deposition gas (e.g.,an oxygen gas, an argon gas, or a mixed gas containing oxygen at 50 vol% or higher) and controlled to higher than or equal to 0.01 Pa and lowerthan or equal to 100 Pa, preferably higher than or equal to 0.1 Pa andlower than or equal to 10 Pa. Here, discharge starts by application of avoltage at a certain value or higher to the target 1130, and plasma isobserved. Note that the magnetic field over the target 1130 forms ahigh-density plasma region. In the high-density plasma region, thedeposition gas is ionized, so that an ion 1101 is generated. Examples ofthe ion 1101 include an oxygen cation (O⁺) and an argon cation (Ar⁺).

The ion 1101 is accelerated to the target 1130 side by an electricfield, and collides with the target 1130 eventually. At this time, apellet 1100 a and a pellet 1100 b which are flat-plate-like orpellet-like sputtered particles are separated and sputtered from thecleavage plane. Note that structures of the pellet 1100 a and the pellet1100 b may be distorted by an impact of collision of the ion 1101.

The pellet 1100 a is a flat-plate-like or pellet-like sputtered particlehaving a triangle plane, e.g., a regular triangle plane. The pellet 1100b is a flat-plate-like or pellet-like sputtered particle having ahexagon plane, e.g., regular hexagon plane. Note that flat-plate-like orpellet-like sputtered particles such as the pellet 1100 a and the pellet1100 b are collectively called pellets 1100. The shape of a flat planeof the pellet 1100 is not limited to a triangle or a hexagon. Forexample, the flat plane may have a shape formed by combining greaterthan or equal to 2 and less than or equal to 6 triangles. For example, asquare (rhombus) is formed by combining two triangles (regulartriangles) in some cases.

The thickness of the pellet 1100 is determined depending on the kind ofthe deposition gas and the like. The thicknesses of the pellets 1100 arepreferably uniform; the reasons thereof are described later. Inaddition, the sputtered particle preferably has a pellet shape with asmall thickness as compared to a dice shape with a large thickness.

The pellet 1100 receives charge when passing through the plasma, so thatside surfaces of the pellet 1100 are negatively or positively charged insome cases. The pellet 1100 includes an oxygen atom on its side surface,and the oxygen atom may be negatively charged. For example, a case inwhich the pellet 1100 a includes, on its side surfaces, oxygen atomsthat are negatively charged is illustrated in FIG. 42. As in this view,when the side surfaces are charged in the same polarity, charges repeleach other, and accordingly, the pellet can maintain a flat-plate shape.In the case where a CAAC-OS is an In—Ga—Zn oxide, there is a possibilitythat an oxygen atom bonded to an indium atom is negatively charged.There is another possibility that an oxygen atom bonded to an indiumatom, a gallium atom, or a zinc atom is negatively charged.

As shown in FIG. 40A, the pellet 1100 flies like a kite in plasma andflutters up to the substrate 1120. Since the pellets 1100 are charged,when the pellet 1100 gets close to a region where another pellet 1100has already been deposited, repulsion is generated. Here, above thesubstrate 1120, a magnetic field is generated in a direction parallel toa top surface of the substrate 1120. A potential difference is givenbetween the substrate 1120 and the target 1130, and accordingly, currentflows from the substrate 1120 toward the target 1130. Thus, the pellet1100 is given a force (Lorentz force) on the top surface of thesubstrate 1120 by an effect of the magnetic field and the current (seeFIG. 43). This is explainable with Fleming's left-hand rule. In order toincrease a force applied to the pellet 1100, it is preferable toprovide, on the top surface, a region where the magnetic field in adirection parallel to the top surface of the substrate 1120 is 10 G orhigher, preferably 20 G or higher, further preferably 30 G or higher,still further preferably 50 G or higher. Alternatively, it is preferableto provide, on the top surface, a region where the magnetic field in adirection parallel to the top surface of the substrate is 1.5 times orhigher, preferably twice or higher, further preferably 3 times orhigher, still further preferably 5 times or higher as high as themagnetic field in a direction perpendicular to the top surface of thesubstrate 1120.

Furthermore, the substrate 1120 is heated, and resistance such asfriction between the pellet 1100 and the substrate 1120 is low. As aresult, as illustrated in FIG. 44A, the pellet 1100 glides above thesurface of the substrate 1120. The glide of the pellet 1100 is caused ina state where the flat plane faces the substrate 1120. Then, asillustrated in FIG. 44B, when the pellet 1100 reaches the side surfaceof another pellet 1100 that has been already deposited, the sidesurfaces of the pellets 1100 are bonded. At this time, the oxygen atomon the side surface of the pellet 1100 is released. With the releasedoxygen atom, oxygen vacancy in a CAAC-OS is filled in some cases; thus,the CAAC-OS has a low density of defect states.

Further, the pellet 1100 is heated on the substrate 1120, whereby atomsare rearranged, and the structure distortion caused by the collision ofthe ion 1101 can be reduced. The pellet 1100 whose structure distortionis reduced is substantially single crystal. Even when the pellets 1100are heated after being bonded, expansion and contraction of the pellet1100 itself hardly occur, which is caused by turning the pellet 1100into substantially single crystal. Thus, formation of defects such as agrain boundary due to expansion of a space between the pellets 1100 canbe prevented, and accordingly, generation of crevasses can be prevented.Further, the space is filled with elastic metal atoms and the like,whereby the elastic metal atoms have a function, like a highway, ofjointing side surfaces of the pellets 1100 which are not aligned witheach other.

It is considered that as shown in such a model, the pellets 1100 aredeposited over the substrate 1120. Thus, a CAAC-OS film can be depositedeven when a surface over which a film is formed (film formation surface)does not have a crystal structure, which is different from filmdeposition by epitaxial growth. For example, even when a surface (filmformation surface) of the substrate 1120 has an amorphous structure, aCAAC-OS film can be formed.

Further, it is found that in formation of the CAAC-OS, the pellets 1100are arranged in accordance with a surface shape of the substrate 1120that is the film formation surface even when the film formation surfacehas unevenness besides a flat surface. For example, in the case wherethe surface of the substrate 1120 is flat at the atomic level, thepellets 1100 are arranged so that flat planes parallel to the a-b planeface downwards; thus, a layer with a uniform thickness, flatness, andhigh crystallinity is formed. By stacking n layers (n is a naturalnumber), the CAAC-OS can be obtained (see FIG. 40B).

In the case where the top surface of the substrate 1120 has unevenness,a CAAC-OS where n layers (n is a natural number) in each of which thepellets 1100 are arranged along a convex surface are stacked is formed.Since the substrate 1120 has unevenness, a gap is easily generatedbetween in the pellets 1100 in the CAAC-OS in some cases. Note thatowing to intermolecular force, the pellets 1100 are arranged so that agap between the pellets is as small as possible even on the unevennesssurface. Therefore, even when the formation surface has unevenness, aCAAC-OS with high crystallinity can be formed (see FIG. 40C).

As a result, laser crystallization is not needed for formation of aCAAC-OS, and a uniform film can be formed even over a large-sized glasssubstrate.

Since the CAAC-OS film is deposited in accordance with such a model, thesputtered particle preferably has a pellet shape with a small thickness.Note that in the case where the sputtered particle has a dice shape witha large thickness, planes facing the substrate 1120 are not uniform andthus, the thickness and the orientation of the crystals cannot beuniform in some cases.

According to the deposition model described above, a CAAC-OS with highcrystallinity can be formed even on a film formation surface with anamorphous structure.

Further, formation of a CAAC-OS can be described with a deposition modelincluding a zinc oxide particle besides the pellet 1100.

The zinc oxide particle reaches the substrate 1120 before the pellet1100 does because the zinc oxide particle is smaller than the pellet1100 in mass. On the surface of the substrate 1120, crystal growth ofthe zinc oxide particle preferentially occurs in the horizontaldirection, so that a thin zinc oxide layer is formed. The zinc oxidelayer has c-axis alignment. Note that c-axes of crystals in the zincoxide layer are aligned in the direction parallel to a normal vector ofthe substrate 1120. The zinc oxide layer serves as a seed layer thatmakes a CAAC-OS grow and thus has a function of increasing crystallinityof the CAAC-OS. The thickness of the zinc oxide layer is greater than orequal to 0.1 nm and less than or equal to 5 nm, mostly greater than orequal to 1 nm and less than or equal to 3 nm. Since the zinc oxide layeris sufficiently thin, a grain boundary is hardly observed.

Thus, in order to deposit a CAAC-OS with high crystallinity, a targetincluding zinc at a proportion higher than that of the stoichiometriccomposition is preferably used.

An nc-OS can be understood with a deposition model illustrated in FIG.41. Note that a difference between FIG. 41 and FIG. 40A lies only in thefact that whether the substrate 1120 is heated or not.

Thus, the substrate 1120 is not heated, and a resistance such asfriction between the pellet 1100 and the substrate 1120 is high. As aresult, the pellets 1100 cannot glide on the surface of the substrate1120 and are stacked randomly, thereby forming an nc-OS.

<Cleavage Plane>

A cleavage plane that has been mentioned in the deposition model of theCAAC-OS will be described below.

First, a cleavage plane of the target is described with reference toFIGS. 45A and 45B. FIGS. 45A and 45B show the crystal structure ofInGaZnO₄. Note that FIG. 45A shows the structure of the case where anInGaZnO₄ crystal is observed from a direction parallel to the b-axiswhen the c-axis is in an upward direction. Furthermore, FIG. 45B showsthe structure of the case where the InGaZnO₄ crystal is observed from adirection parallel to the c-axis.

Energy needed for cleavage at each of crystal planes of the InGaZnO₄crystal is calculated by the first principles calculation. Note that a“pseudopotential” and density functional theory program (CASTEP) usingthe plane wave basis are used for the calculation. Note that anultrasoft type pseudopotential is used as the pseudopotential. Further,GGA/PBE is used as the functional. Cut-off energy is 400 eV.

Energy of a structure in an initial state is obtained after structuraloptimization including a cell size is performed. Further, energy of astructure after the cleavage at each plane is obtained after structuraloptimization of atomic arrangement is performed in a state where thecell size is fixed.

On the basis of the structure of the InGaZnO₄ crystal in FIGS. 45A and45B, a structure cleaved at any one of a first plane, a second plane, athird plane, and a fourth plane is formed and subjected to structuraloptimization calculation in which the cell size is fixed. Here, thefirst plane is a crystal plane between a Ga—Zn—O layer and an In—O layerand is parallel to the (001) plane (the a-b plane) (see FIG. 45A). Thesecond plane is a crystal plane between a Ga—Zn—O layer and a Ga—Zn—Olayer and is parallel to the (001) plane (the a-b plane) (see FIG. 45A).The third plane is a crystal plane parallel to the (110) plane (see FIG.45B). The fourth plane is a crystal plane parallel to the (100) plane(the b-c plane) (see FIG. 45B).

Under the above conditions, the energy of the structure at each planeafter the cleavage is calculated. Next, a difference between the energyof the structure after the cleavage and the energy of the structure inthe initial state is divided by the area of the cleavage plane; thus,cleavage energy which serves as a measure of easiness of cleavage ateach plane is calculated. Note that the energy of a structure indicatesenergy obtained in such a manner that electronic kinetic energy ofelectrons included in the structure and interactions between atomsincluded in the structure, between the atom and the electron, andbetween the electrons are considered.

As calculation results, the cleavage energy of the first plane was 2.60J/m², that of the second plane was 0.68 J/m², that of the third planewas 2.18 J/m², and that of the fourth plane was 2.12 J/m² (see Table 1).

TABLE 1 Cleavage energy [J/m²] First plane 2.60 Second plane 0.68 Thirdplane 2.18 Fourth plane 2.12

From the calculations, in the structure of the InGaZnO₄ crystal in FIGS.45A and 45B, the cleavage energy of the second plane is the lowest. Inother words, a plane between a Ga—Zn—O layer and a Ga—Zn—O layer iscleaved most easily (cleavage plane). Therefore, in this specification,the cleavage plane indicates the second plane, which is a plane wherecleavage is performed most easily.

Since the cleavage plane is the second plane between the Ga—Zn—O layerand the Ga—Zn—O layer, the InGaZnO₄ crystals in FIG. 45A can beseparated at a plane equivalent to two second planes. Therefore, in thecase where an ion or the like is made to collide with a target, awafer-like unit (we call this a pellet) which is cleaved at a plane withthe lowest cleavage energy is thought to be blasted off as the minimumunit. In that case, a pellet of InGaZnO₄ includes three layers: aGa—Zn—O layer, an In—O layer, and a Ga—Zn—O layer.

The cleavage energies of the third plane (crystal plane parallel to the(110) plane) and the fourth plane (crystal plane parallel to the (100)plane (the b-c plane)) are lower than that of the first plane (crystalplane between the Ga—Zn—O layer and the In—O layer and plane that isparallel to the (001) plane (the a-b plane)), which suggests that mostof the flat planes of the pellets have triangle shapes or hexagonalshapes.

Next, through classical molecular dynamics calculation, on theassumption of an InGaZnO₄ crystal having a homologous structure as atarget, a cleavage plane in the case where the target is sputtered usingargon (Ar) or oxygen (O) is examined. FIG. 46A shows a cross-sectionalstructure of an InGaZnO₄ crystal (2688 atoms) used for the calculation,and FIG. 46B shows a top structure thereof. Note that a fixed layer inFIG. 46A prevents the positions of the atoms from moving. A temperaturecontrol layer in FIG. 46A is a layer whose temperature is constantly setto fixed temperature (300 K).

For the classical molecular dynamics calculation, Materials Explorer 5.0manufactured by Fujitsu Limited. is used. Note that the initialtemperature, the cell size, the time step size, and the number of stepsare set to be 300 K, a certain size, 0.01 fs, and ten million,respectively. In calculation, an atom to which an energy of 300 eV isapplied is made to enter a cell from a direction perpendicular to thea-b plane of the InGaZnO₄ crystal under the conditions.

FIG. 47A shows atomic order when 99.9 picoseconds have passed afterargon enters the cell including the InGaZnO₄ crystal in FIGS. 46A and46B. FIG. 47B shows atomic order when 99.9 picoseconds have passed afteroxygen enters the cell. Note that in FIGS. 47A and 47B, part of thefixed layer in FIG. 46A is omitted.

According to FIG. 47A, in a period from entry of argon into the cell towhen 99.9 picoseconds have passed, a crack is formed from the cleavageplane corresponding to the second plane in FIG. 45A. Thus, in the casewhere argon collides with the InGaZnO₄ crystal and the uppermost surfaceis the second plane (the zero-th), a large crack is found to be formedin the second plane (the second).

On the other hand, according to FIG. 47B, in a period from entry ofoxygen into the cell to when 99.9 picoseconds have passed, a crack isfound to be formed from the cleavage plane corresponding to the secondplane in FIG. 45A. Note that in the case where oxygen collides with thecell, a large crack is found to be formed in the second plane (thefirst) of the InGaZnO₄ crystal.

Accordingly, it is found that an atom (ion) collides with a targetincluding an InGaZnO₄ crystal having a homologous structure from theupper surface of the target, the InGaZnO₄ crystal is cleaved along thesecond plane, and a flat-plate-like sputtered particle (pellet) isseparated. It is also found that the pellet formed in the case whereoxygen collides with the cell is smaller than that formed in the casewhere argon collides with the cell.

The above calculation suggests that the separated pellet includes adamaged region. In some cases, the damaged region included in the pelletcan be repaired in such a manner that a defect caused by the damagereacts with oxygen.

Here, difference in size of the pellet depending on atoms which are madeto collide is studied.

FIG. 48A shows trajectories of the atoms from 0 picosecond to 0.3picoseconds after argon enters the cell including the InGaZnO₄ crystalin FIGS. 46A and 46B. Accordingly, FIG. 48A corresponds to a period fromFIGS. 46A and 46B to FIG. 47A.

According to FIG. 48A, when argon collides with gallium (Ga) of thefirst layer (Ga—Zn—O layer), gallium collides with zinc (Zn) of thethird layer (Ga—Zn—O layer) and then, zinc reaches the vicinity of thesixth layer (Ga—Zn—O layer). Note that the argon which collides with thegallium is sputtered to the outside. Accordingly, in the case whereargon collides with the target including the InGaZnO₄ crystal, a crackis thought to be forming in the second plane (the second) in FIG. 46A.

FIG. 48B shows trajectories of the atoms from 0 picosecond to 0.3picoseconds after oxygen enters the cell including the InGaZnO₄ crystalin FIGS. 46A and 46B. Accordingly, FIG. 48B corresponds to a period fromFIGS. 46A and 46B to FIG. 47A.

On the other hand, according to FIG. 48B, when oxygen collides withgallium (Ga) of the first layer (Ga—Zn—O layer), gallium collides withzinc (Zn) of the third layer (Ga—Zn—O layer) and then, zinc does notreach the fifth layer (In—O layer). Note that the oxygen which collideswith the gallium is sputtered to the outside. Accordingly, in the casewhere oxygen collides with the target including the InGaZnO₄ crystal, acrack is thought to be formed in the second plane (the first) in FIG.46A.

This calculation also shows that the InGaZnO₄ crystal with which an atom(ion) collides is separated from the cleavage plane.

In addition, a difference in depth of a crack is examined in view ofconservation laws. The energy conservation law and the law ofconservation of momentum can be represented by the following formula (1)and the following formula (2). Here, E represents energy of argon oroxygen before collision (300 eV), m_(A) represents mass of argon oroxygen, v_(A) represents the speed of argon or oxygen before collision,v′_(A) represents the speed of argon or oxygen after collision, m_(Ga)represents mass of gallium, v_(Ga) represents the speed of galliumbefore collision, and v′_(Ga) represents the speed of gallium aftercollision.E=½m _(A) v _(A) ²+½m _(Ga) v _(Ga) ²  [Formula 1]m _(A) v _(A) +m _(Ga) v _(Ga) =m _(A) v′ _(A) +m _(Ga) v′_(Ga)  [Formula 2]

On the assumption that collision of argon or oxygen is elasticcollision, the relationship among v_(A), v′_(A), v_(Ga), and v′_(Ga) canbe represented by the following formula (3).v′ _(A) −v′ _(Ga)=−(v _(A) −v _(Ga))  [Formula 3]

From the formulae (1), (2), and (3), on the assumption that v_(Ga) is 0,the speed of gallium v′_(Ga) after collision of argon or oxygen can berepresented by the following formula (4).

$\begin{matrix}{v_{Ga}^{\prime} = {{\frac{\sqrt{m_{A}}}{m_{A} + m_{Ga}} \cdot 2}\sqrt{2E}}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack\end{matrix}$

In the formula (4), mass of argon or oxygen is substituted into m_(A),whereby the speeds after collision of the atoms are compared. In thecase where the argon and the oxygen have the same energy beforecollision, the speed of gallium in the case where argon collides withthe gallium was found to be 1.24 times as high as that in the case whereoxygen collides with the gallium. Thus, the energy of the gallium in thecase where argon collides with the gallium is higher than that in thecase where oxygen collides with the gallium by the square of the speed.

The speed (energy) of gallium after collision in the case where argoncollides with the gallium is found to be higher than that in the casewhere oxygen collides with the gallium. Accordingly, it is consideredthat a crack is formed at a deeper position in the case where argoncollides with the gallium than in the case where oxygen collides withthe gallium.

The above calculation shows that separation occurs from the cleavageplane to form a pellet when sputtering is performed using a targetincluding the InGaZnO₄ crystal having a homologous structure. On theother hand, even when sputtering is performed on a region having anotherstructure of a target without the cleavage plane, a pellet is notformed, and a sputtered particle with an atomic-level size which isminuter than a pellet is formed. Because the sputtered particle issmaller than the pellet, the sputtered particle is thought to be removedthrough a vacuum pump connected to a sputtering apparatus. Therefore, amodel in which particles with a variety of sizes and shapes fly to asubstrate and are deposited hardly applies to the case where sputteringis performed using a target including the InGaZnO₄ crystal having ahomologous structure. The model illustrated in FIG. 40A where sputteredpellets are deposited to form a CAAC-OS is a reasonable model.

The CAAC-OS deposited in such a manner has a density substantially equalto that of a single crystal OS. For example, the density of the singlecrystal OS film having a homologous structure of InGaZnO₄ is 6.36 g/cm³,and the density of the CAAC-OS film having substantially the same atomicratio is approximately 6.3 g/cm³.

FIGS. 49A and 49B show atomic order of cross sections of an In—Ga—Znoxide (see FIG. 49A) that is a CAAC-OS deposited by sputtering and atarget thereof (see FIG. 49B). For observation of atomic arrangement, ahigh-angle annular dark field scanning transmission electron microscopy(HAADF-STEM) is used. In the case of observation by HAADF-STEM, theintensity of an image of each atom is proportional to the square of itsatomic number. Therefore, Zn (atomic number: 30) and Ga (atomic number:31), whose atomic numbers are close to each other, are hardlydistinguished from each other. A Hitachi scanning transmission electronmicroscope HD-2700 is used for the HAADF-STEM.

When FIG. 49A and FIG. 49B are compared, it is found that the CAAC-OSand the target each have a homologous structure and atomic order in theCAAC-OS correspond to that in the target. Thus, as illustrated in thedeposition model in FIG. 40A, the crystal structure of the target istransferred, whereby a CAAC-OS is formed.

Next, a relationship between crystallinity and an oxygen-transmittingproperty in the case where the oxide semiconductor film is an In—Ga—Znoxide is described below.

An energy barrier due to movement of excess oxygen (oxygen) in a crystalof an In—Ga—Zn oxide is obtained by calculation. In the calculation,plane-wave basis first-principles calculation software Vienna ab-initiosimulation package (VASP) based on density functional theory is used.GGA-PBE is used as a functional. Cut-off energy of a plane wave is 400eV. The effect of an inner shell electron is included by a projectoraugmented wave (PAW) method.

Here, the ease of movement of excess oxygen (oxygen) through movementpaths 1 to 4 in a crystal of an In—Ga—Zn oxide illustrated in FIG. 22 iscalculated.

The movement path 1 is a path through which excess oxygen (oxygen)bonded to oxygen bonded to three indium atoms and one zinc atom isbonded to adjacent oxygen bonded to three indium atoms and one zincatom. The movement path 2 is a path through which excess oxygen (oxygen)bonded to oxygen bonded to three indium atoms and one gallium atomcrosses a layer containing indium and oxygen and is bonded to adjacentoxygen bonded to three indium atoms and one zinc atom. The movement path3 is a path through which excess oxygen (oxygen) bonded to oxygen bondedto two gallium atoms and one zinc atom is bonded to adjacent oxygenbonded to two zinc atoms and one gallium atom. The movement path 4 is apath through which excess oxygen (oxygen) bonded to oxygen bonded to twogallium atoms and one zinc atom crosses a layer containing gallium,zinc, and oxygen and is bonded to adjacent oxygen bonded to three indiumatoms and one gallium atom.

When the frequency of going over an energy barrier E_(a) per unit timeis referred to as a diffusion frequency R, R can be expressed as thefollowing formula.R=ν·exp[−E _(a)/(k _(B) T)]  [Formula 5]

Note that ν represents the number of heat vibrations of diffusion atoms,k_(B) represents Boltzmann constant, and T represents the absolutetemperature. The diffusion frequency R at 350° C. and 450° C. when 10¹³[1/sec] is applied to ν as Debye frequency is shown in Table 2.

TABLE 2 Energy Diffusion frequency R [1/sec] barrier [eV] 350° C. 450°C. Movement path 1 0.50 9.0 × 10⁸ 3.3 × 10⁹ Movement path 2 1.97  1.2 ×10⁻³  1.9 × 10⁻¹ Movement path 3 0.53 5.2 × 10⁸ 2.0 × 10⁹ Movement path4 0.56 3.0 × 10⁸ 1.3 × 10⁹

As shown in Table 2, the movement path 2 across the layer containingindium and oxygen has a higher energy barrier than the other movementpaths. This indicates that movement of excess oxygen (oxygen) in thec-axis direction is less likely to occur in a crystal of an In—Ga—Znoxide. In other words, in the case where crystals have c-axis alignmentand the c-axes are aligned in a direction substantially perpendicular toa formation surface or a top surface, like CAAC-OS, movement of excessoxygen (oxygen) is less likely to occur in the direction substantiallyperpendicular to the formation surface or the top surface.

The structure and method described in this embodiment can be implementedby being combined as appropriate with any of the other structures andmethods described in the other embodiments.

Embodiment 3

In this embodiment, oxygen vacancy of an oxide semiconductor film isdescribed in detail below.

<(1) Ease of Formation and Stability of V_(o)H>

In the case where an oxide semiconductor film (hereinafter referred toas IGZO) is a complete crystal, H preferentially diffuses along the a-bplane at a room temperature. In heat treatment at 450° C., H diffusesalong the a-b plane and in the c-axis direction. Here, description ismade on whether H easily enters oxygen vacancy V_(o) if the oxygenvacancy V_(o) exists in IGZO. A state in which H is in oxygen vacancyV_(o) is referred to as V_(o)H.

An InGaZnO₄ crystal model shown in FIG. 23 was used for calculation. Theactivation barrier (E_(a)) along the reaction path where H in V_(o)H isreleased from V_(o) and bonded to oxygen was calculated by a nudgedelastic band (NEB) method. The calculation conditions are shown in Table3.

TABLE 3 Software VASP Calculation method NEB method Functional GGA-PBEPseudopotential PAW Cut-off energy 500 eV K points 2 × 2 × 3

In the InGaZnO₄ crystal model, there are oxygen sites 1 to 4 as shown inFIG. 23 which differ from each other in metal elements bonded to oxygenand the number of bonded metal elements. Here, calculation was made onthe oxygen sites 1 and 2 in which oxygen vacancy V_(o) is easily formed.

First, calculation was made on the oxygen site in which oxygen vacancyV_(o) is easily formed: an oxygen site 1 that was bonded to three Inatoms and one Zn atom.

FIG. 24A shows a model in the initial state and FIG. 24B shows a modelin the final state. FIG. 25 shows the calculated activation barrier(E_(a)) in the initial state and the final state. Note that here, theinitial state refers to a state in which H exists in oxygen vacancyV_(o) (V_(o)H), and the final state refers to a structure includingoxygen vacancy V_(o) and a state in which H is bonded to oxygen bondedto one Ga atom and two Zn atoms (H—O).

From the calculation results, bonding of H in oxygen vacancy V_(o) toanother oxygen atom needs an energy of approximately 1.52 eV, whileentry of H bonded to O into oxygen vacancy V_(o) needs an energy ofapproximately 0.46 eV.

Reaction frequency (F) was calculated with use of the activationbarriers (E_(a)) obtained by the calculation and Formula 6. In Formula6, k_(B) represents the Boltzmann constant and T represents the absolutetemperature.

$\begin{matrix}{\Gamma = {v\;{\exp\left( {- \frac{E_{a}}{k_{B}T}} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 6} \right\rbrack\end{matrix}$

The reaction frequency at 350° C. was calculated on the assumption thatthe frequency factor ν=10¹³ [1/sec]. The frequency of H transfer fromthe model shown in FIG. 24A to the model shown in FIG. 24B was 5.52×10⁰[1/sec], whereas the frequency of H transfer from the model shown inFIG. 24B to the model shown in FIG. 24A was 1.82×10⁹ [1/sec]. Thissuggests that H diffusing in IGZO is likely to form V_(o)H if oxygenvacancy V_(o) exists in the neighborhood, and H is unlikely to bereleased from the oxygen vacancy V_(o) once V_(o)H is formed.

Next, calculation was made on the oxygen site in which oxygen vacancyV_(o) is easily formed: an oxygen site 2 that was bonded to one Ga atomand two Zn atoms.

FIG. 26A shows a model in the initial state and FIG. 26B shows a modelin the final state. FIG. 27 shows the calculated activation barrier(E_(a)) in the initial state and the final state. Note that here, theinitial state refers to a state in which H exists in oxygen vacancyV_(o) (V_(o)H), and the final state refers to a structure includingoxygen vacancy V_(o) and a state in which H is bonded to oxygen bondedto one Ga atom and two Zn atoms (H—O).

From the calculation results, bonding of H in oxygen vacancy V_(o) toanother oxygen atom needs an energy of approximately 1.75 eV, whileentry of H bonded to O in oxygen vacancy V_(o) needs an energy ofapproximately 0.35 eV.

Reaction frequency (F) was calculated with use of the activationbarriers (E_(a)) obtained by the calculation and Formula 6.

The reaction frequency at 350° C. was calculated on the assumption thatthe frequency factor ν=10¹³ [1/sec]. The frequency of H transfer fromthe model shown in FIG. 26A to the model shown in FIG. 26B was 7.53×10⁻²[1/sec], whereas the frequency of H transfer from the model shown inFIG. 26B to the model shown in FIG. 26A was 1.44×10¹⁰ [1/sec]. Thissuggests that H is unlikely to be released from the oxygen vacancy V_(o)once V_(o)H is formed.

From the above results, it was found that H in IGZO easily diffused inannealing and if oxygen vacancy V_(o) existed, H was likely to enter theoxygen vacancy V_(o) to be V_(o)H.

<(2) Transition Level of V_(o)H>

The calculation by the NEB method, which was described in <(1) Ease offormation and stability of V_(o)H>, indicates that in the case whereoxygen vacancy V_(o) and H exist in IGZO, the oxygen vacancy V_(o) and Heasily form V_(o)H and V_(o)H is stable. To determine whether V_(o)H isrelated to a carrier trap, the transition level of V_(o)H wascalculated.

The model used for calculation is an InGaZnO₄ crystal model (112 atoms).V_(o)H models of the oxygen sites 1 and 2 shown in FIG. 25 were made tocalculate the transition levels. The calculation conditions are shown inTable 4.

TABLE 4 Software VASP Model InGaZnO₄ crystal model (112 atoms)Functional HSE06 Mixture ratio of exchange terms 0.25 PseudopotentialGGA-PBE Cut-off energy 800 eV K points 1 × 1 × 1

The mixture ratio of exchange terms was adjusted to have a band gapclose to the experimental value. As a result, the band gap of theInGaZnO₄ crystal model without defects was 3.08 eV that is close to theexperimental value, 3.15 eV.

The transition level (ε(q/q′)) of a model having defect D can becalculated by the following Formula 7. Note that ΔE(D^(q)) representsthe formation energy of defect D at charge q, which is calculated byFormula 8.

$\begin{matrix}{\mspace{79mu}{{ɛ\left( {q/q^{\prime}} \right)} = \frac{{\Delta\;{E\left( D^{q} \right)}} - {\Delta\;{E\left( D^{q^{\prime}} \right)}}}{q^{\prime} - q}}} & \left\lbrack {{Formula}\mspace{14mu} 7} \right\rbrack \\{{\Delta\;{E\left( D^{q} \right)}} = {{E_{tot}\left( D^{q} \right)} - {E_{tot}({bulk})} + {\sum\limits_{i}{\Delta\; n_{i}\mu_{i}}} + {q\left( {ɛ_{VBM} + {\Delta\; V_{q}} + E_{F}} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 8} \right\rbrack\end{matrix}$

In Formulae 7 and 8, E_(tot)(D^(q)) represents the total energy of themodel having defect D at the charge q in, E_(tot)(bulk) represents thetotal energy in a model without defects (complete crystal), Δn_(i)represents a change in the number of atoms i contributing to defects,μ_(i) represents the chemical potential of atom i, ε_(VBM) representsthe energy of the valence band maximum in the model without defects,ΔV_(q) represents the correction term relating to the electrostaticpotential, and E_(F) represents the Fermi energy.

FIG. 28 shows the transition levels of V_(o)H obtained from the aboveformulae. The numbers in FIG. 28 represent the depth from the conductionband minimum. In FIG. 28, the transition level of V_(o)H in the oxygensite 1 is at 0.05 eV from the conduction band minimum, and thetransition level of V_(o)H in the oxygen site 2 is at 0.11 eV from theconduction band minimum. Therefore, these V_(o)H would be related toelectron traps, that is, V_(o)H was found to behave as a donor. It wasalso found that IGZO including V_(o)H had conductivity.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 4

In this embodiment, an example of a display device that includes any ofthe transistors described in the embodiment above is described belowwith reference to FIG. 29, FIG. 30, and FIG. 31.

FIG. 29 is a top view of an example of a display device. A displaydevice 700 illustrated in FIG. 29 includes a pixel portion 702 providedover a first substrate 701; a source driver circuit portion 704 and agate driver circuit portion 706 provided over the first substrate 701; asealant 712 provided to surround the pixel portion 702, the sourcedriver circuit portion 704, and the gate driver circuit portion 706; anda second substrate 705 provided to face the first substrate 701. Thefirst substrate 701 and the second substrate 705 are sealed with thesealant 712. That is, the pixel portion 702, the source driver circuitportion 704, and the gate driver circuit portion 706 are sealed with thefirst substrate 701, the sealant 712, and the second substrate 705.Although not illustrated in FIG. 29, a display element is providedbetween the first substrate 701 and the second substrate 705.

In the display device 700, a flexible printed circuit (FPC) terminalportion 708 electrically connected to the pixel portion 702, the sourcedriver circuit portion 704, and the gate driver circuit portion 706 isprovided in a region different from the region which is surrounded bythe sealant 712 and positioned over the first substrate 701.Furthermore, an FPC 716 is connected to the FPC terminal portion 708,and a variety of signals and the like are supplied to the pixel portion702, the source driver circuit portion 704, and the gate driver circuitportion 706 through the FPC 716. Furthermore, a signal line 710 isconnected to the pixel portion 702, the source driver circuit portion704, the gate driver circuit portion 706, and the FPC terminal portion708. Various signals and the like are applied to the pixel portion 702,the source driver circuit portion 704, the gate driver circuit portion706, and the FPC terminal portion 708 via the signal line 710 from theFPC 716.

A plurality of gate driver circuit portions 706 may be provided in thedisplay device 700. An example of the display device 700 in which thesource driver circuit portion 704 and the gate driver circuit portion706 are formed over the first substrate 701 where the pixel portion 702is also formed is described; however, the structure is not limitedthereto. For example, only the gate driver circuit portion 706 may beformed over the first substrate 701 or only the source driver circuitportion 704 may be formed over the first substrate 701. In this case, asubstrate where a source driver circuit, a gate driver circuit, or thelike is formed (e.g., a driver-circuit substrate formed using asingle-crystal semiconductor film or a polycrystalline semiconductorfilm) may be mounted on the first substrate 701. Note that there is noparticular limitation on the method of connecting a separately prepareddriver circuit substrate, and a chip on glass (COG) method, a wirebonding method, or the like can be used.

The pixel portion 702, the source driver circuit portion 704, and thegate driver circuit portion 706 included in the display device 700include a plurality of transistors. As the plurality of transistors, anyof the transistors that are the semiconductor devices of embodiments ofthe present invention can be used.

The display device 700 can include any of a variety of elements. Theelement includes, for example, at least one of a liquid crystal element,an electroluminescence (EL) element (e.g., an EL element includingorganic and inorganic materials, an organic EL element, or an inorganicEL element), an LED (e.g., a white LED, a red LED, a green LED, or ablue LED), a transistor (a transistor that emits light depending oncurrent), an electron emitter, electronic ink, an electrophoreticelement, a grating light valve (GLV), a plasma display panel (PDP), adisplay element using micro electro mechanical system (MEMS), a digitalmicromirror device (DMD), a digital micro shutter (DMS), MIRASOL(registered trademark), an interferometric modulator display (IMOD)element, a MEMS shutter display element, an optical-interference-typeMEMS display element, an electrowetting element, a piezoelectric ceramicdisplay, and a display element including a carbon nanotube. Other thanthe above, display media whose contrast, luminance, reflectivity,transmittance, or the like is changed by an electrical or magneticeffect may be included. Examples of display devices having EL elementsinclude an EL display. Examples of display devices including electronemitters include a field emission display (FED) and an SED-type flatpanel display (SED: surface-conduction electron-emitter display).Examples of display devices including liquid crystal elements include aliquid crystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). An example of a display device including electronicink or electrophoretic elements is electronic paper. In the case of atransflective liquid crystal display or a reflective liquid crystaldisplay, some of or all of pixel electrodes function as reflectiveelectrodes. For example, some or all of pixel electrodes are formed toinclude aluminum, silver, or the like. In such a case, a memory circuitsuch as an SRAM can be provided under the reflective electrodes, leadingto lower power consumption.

As a display method in the display device 700, a progressive method, aninterlace method, or the like can be employed. Furthermore, colorelements controlled in a pixel at the time of color display are notlimited to three colors: R, G, and B (R, G, and B correspond to red,green, and blue, respectively). For example, four pixels of the R pixel,the G pixel, the B pixel, and a W (white) pixel may be included.Alternatively, a color element may be composed of two colors among R, G,and B as in PenTile layout. The two colors may differ among colorelements. Alternatively, one or more colors of yellow, cyan, magenta,and the like may be added to RGB. Further, the size of a display regionmay be different depending on respective dots of the color components.Embodiments of the disclosed invention are not limited to a displaydevice for color display; the disclosed invention can also be applied toa display device for monochrome display.

A coloring layer (also referred to as a color filter) may be used inorder to obtain a full-color display device in which white light (W) fora backlight (e.g., an organic EL element, an inorganic EL element, anLED, or a fluorescent lamp) is used. As the coloring layer, red (R),green (G), blue (B), yellow (Y), or the like may be combined asappropriate, for example. With the use of the coloring layer, highercolor reproducibility can be obtained than in the case without thecoloring layer. In this case, by providing a region with the coloringlayer and a region without the coloring layer, white light in the regionwithout the coloring layer may be directly utilized for display. Bypartly providing the region without the coloring layer, a decrease inluminance due to the coloring layer can be suppressed, and 20% to 30% ofpower consumption can be reduced in some cases when an image isdisplayed brightly. Note that in the case where full-color display isperformed using a self-luminous element such as an organic EL element oran inorganic EL element, elements may emit light of their respectivecolors R, G, B, Y, and W. By using a self-luminous element, powerconsumption can be further reduced as compared to the case of using thecoloring layer in some cases.

In this embodiment, a structure including a liquid crystal element andan EL element as display elements is described with reference to FIG. 30and FIG. 31. Note that FIG. 30 is a cross-sectional view along thedashed-dotted line Q-R shown in FIG. 29 and shows a structure includinga liquid crystal element as a display element, whereas FIG. 31 is across-sectional view along the dashed-dotted line Q-R shown in FIG. 29and shows a structure including an EL element as a display element.

Common portions between FIG. 30 and FIG. 31 are described first, andthen different portions are described.

<Common Portions in Display Devices>

The display device 700 illustrated in FIG. 30 and FIG. 31 include a leadwiring portion 711, the pixel portion 702, the source driver circuitportion 704, and the FPC terminal portion 708. Note that the lead wiringportion 711 includes a signal line 710. The pixel portion 702 includes atransistor 750 and a capacitor 790. The source driver circuit portion704 includes a transistor 752.

Any of the transistors described above can be used as the transistors750 and 752.

The transistors used in this embodiment each include an oxidesemiconductor film which is highly purified and in which formation ofoxygen vacancies is suppressed. In the transistor, the current in an offstate (off-state current) can be made small. Accordingly, an electricalsignal such as an image signal can be held for a longer period, and awriting interval can be set longer in an on state. Accordingly,frequency of refresh operation can be reduced, which leads to an effectof suppressing power consumption.

In addition, the transistor used in this embodiment can have relativelyhigh field-effect mobility and thus is capable of high speed operation.For example, with such a transistor which can operate at high speed usedfor a liquid crystal display device, a switching transistor in a pixelportion and a driver transistor in a driver circuit portion can beformed over one substrate. That is, a semiconductor device formed usinga silicon wafer or the like is not additionally needed as a drivercircuit, by which the number of components of the semiconductor devicecan be reduced. In addition, the transistor which can operate at highspeed can be used also in the pixel portion, whereby a high-qualityimage can be provided.

The capacitor 790 includes a dielectric between a pair of electrodes.Specifically, a conductive film which is formed using the same step as aconductive film functioning as a gate electrode of the transistor 750 isused as one electrode of the capacitor 790, and a conductive filmfunctioning as a source electrode or a drain electrode of the transistor750 is used as the other electrode of the capacitor 790. Furthermore, aninsulating film functioning as a gate insulating film of the transistor750 is used as the dielectric between the pair of electrodes.

In FIG. 30 and FIG. 31, insulating films 764, 766, and 768 and aplanarization insulating film 770 are formed over the transistor 750,the transistor 752, and the capacitor 790.

The insulating films 764, 766, and 768 can be formed using materials andmethods similar to those of the insulating films 114, 116, and 118described in the above embodiment, respectively. The planarizationinsulating film 770 can be formed using a heat-resistant organicmaterial, such as a polyimide resin, an acrylic resin, a polyimide amideresin, a benzocyclobutene resin, a polyamide resin, or an epoxy resin.Note that the planarization insulating film 770 may be formed bystacking a plurality of insulating films formed from these materials.Alternatively, a structure without the planarization insulating film 770may be employed.

The signal line 710 is formed in the same process as conductive filmsfunctioning as a source electrode and a drain electrode of thetransistor 750 or 752. Note that the signal line 710 may be formed usinga conductive film which is formed in a different process as a sourceelectrode and a drain electrode of the transistor 750 or 752, e.g., aconductive film functioning as a gate electrode may be used. In the casewhere the signal line 710 is formed using a material including a copperelement, signal delay or the like due to wiring resistance is reduced,which enables display on a large screen.

The FPC terminal portion 708 includes a connection electrode 760, ananisotropic conductive film 780, and the FPC 716. Note that theconnection electrode 760 is formed in the same process as conductivefilms functioning as a source electrode and a drain electrode of thetransistor 750 or 752. The connection electrode 760 is electricallyconnected to a terminal included in the FPC 716 through the anisotropicconductive film 780.

For example, a glass substrate can be used as the first substrate 701and the second substrate 705. A flexible substrate may be used as thefirst substrate 701 and the second substrate 705. Examples of theflexible substrate include a plastic substrate.

A structure body 778 is provided between the first substrate 701 and thesecond substrate 705. The structure body 778 is a columnar spacerobtained by selective etching of an insulating film and provided tocontrol the distance (cell gap) between the first substrate 701 and thesecond substrate 705. Note that a spherical spacer may be used as thestructure body 778. Although the structure in which the structure body778 is provided on the first substrate 701 side is described as anexample in this embodiment, one embodiment of the present invention isnot limited thereto. For example, a structure in which the structurebody 778 is provided on the second substrate 705 side, or a structure inwhich both of the first substrate 701 and the second substrate 705 areprovided with the structure body 778 may be employed.

Furthermore, a light-blocking film 738 functioning as a black matrix, acoloring film 736 functioning as a color filter, and an insulating film734 in contact with the light-blocking film 738 and the coloring film736 are provided on the second substrate 705 side.

<Structure Example of Display Device Using Liquid Crystal Element asDisplay Element>

The display device 700 illustrated in FIG. 30 includes a liquid crystalelement 775. The liquid crystal element 775 includes a conductive film772, a conductive film 774, and a liquid crystal layer 776. Theconductive film 774 is provided on the second substrate 705 side andfunctions as a counter electrode. The display device 700 in FIG. 30 iscapable of displaying an image in such a manner that transmission ornon-transmission is controlled by change in the alignment state of theliquid crystal layer 776 depending on a voltage applied to theconductive film 772 and the conductive film 774.

The conductive film 772 is connected to the conductive films functioningas a source electrode and a drain electrode included in the transistor750. The conductive film 772 is formed over the planarization insulatingfilm 770 to function as a pixel electrode, i.e., one electrode of thedisplay element. The conductive film 772 has a function of a reflectiveelectrode. The display device 700 in FIG. 30 is what is called areflective color liquid crystal display device in which external lightis reflected by the conductive film 772 to display an image through thecoloring film 736.

A conductive film that transmits visible light or a conductive film thatreflects visible light can be used for the conductive film 772. Forexample, a material including one kind selected from indium (In), zinc(Zn), and tin (Sn) is preferably used for the conductive film thattransmits visible light. For example, a material including aluminum orsilver may be used for the conductive film that reflects visible light.In this embodiment, the conductive film that reflects visible light isused for the conductive film 772.

In the case where a conductive film which reflects visible light is usedas the conductive film 772, the conductive film may have a stacked-layerstructure. For example, a 100-nm-thick aluminum film is formed as thebottom layer, and a 30-nm-thick silver alloy film (e.g., an alloy filmincluding silver, palladium, and copper) is formed as the top layer.Such a structure makes it possible to obtain the following effects.

(1) Adhesion between the base film and the conductive film 772 can beimproved.

(2) The aluminum film and the silver alloy film can be collectivelyetched depending on a chemical solution.

(3) The conductive film 772 can have a favorable cross-sectional shape(e.g., a tapered shape).

The reason for (3) is as follows: the etching rate of the aluminum filmwith the chemical solution is higher than that of the copper alloy film,or etching of the aluminum film that is the bottom layer is developedfaster than that of the silver alloy film because when the aluminum filmthat is the bottom layer is exposed after the etching of the silveralloy film that is the top layer, electrons are extracted from metalthat is less noble than the silver alloy film, i.e., aluminum that ismetal having a high ionization tendency, and thus etching of the silveralloy film is suppressed.

Note that projections and depressions are provided in part of theplanarization insulating film 770 of the pixel portion 702 in thedisplay device 700 in FIG. 30. The projections and depressions can beformed in such a manner that the planarization insulating film 770 isformed using an organic resin film or the like, and projections anddepressions are formed on the surface of the organic resin film. Theconductive film 772 functioning as a reflective electrode is formedalong the projections and depressions. Therefore, when external light isincident on the conductive film 772, the light is reflected diffusely atthe surface of the conductive film 772, whereby visibility can beimproved.

Note that the display device 700 illustrated in FIG. 30 is a reflectivecolor liquid crystal display device given as an example, but a displaytype is not limited thereto. For example, a transmissive color liquidcrystal display device in which the conductive film 772 is a conductivefilm that transmits visible light may be used. In the case of atransmissive color liquid crystal display device, projections anddepressions are not necessarily provided on the planarization insulatingfilm 770.

Although not illustrated in FIG. 30, an alignment film may be providedon a side of the conductive film 772 in contact with the liquid crystallayer 776 and on a side of the conductive film 774 in contact with theliquid crystal layer 776. Although not illustrated in FIG. 30, anoptical member (an optical substrate) and the like such as a polarizingmember, a retardation member, or an anti-reflection member may beprovided as appropriate. For example, circular polarization may beemployed by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a sidelight, or the like may be used as a lightsource.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on conditions.

Alternatively, in the case of employing a horizontal electric fieldmode, a liquid crystal exhibiting a blue phase for which an alignmentfilm is unnecessary may be used. A blue phase is one of liquid crystalphases, which is generated just before a cholesteric phase changes intoan isotropic phase while temperature of cholesteric liquid crystal isincreased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which several weight percent ormore of a chiral material is mixed is used for the liquid crystal layerin order to improve the temperature range. The liquid crystalcomposition which includes liquid crystal exhibiting a blue phase and achiral material has a short response time and optical isotropy. Inaddition, the liquid crystal composition which includes liquid crystalexhibiting a blue phase does not need alignment treatment and has asmall viewing angle dependence. An alignment film does not need to beprovided and rubbing treatment is thus not necessary; accordingly,electrostatic discharge damage caused by the rubbing treatment can beprevented and defects and damage of the liquid crystal display device inthe manufacturing process can be reduced.

In the case where a liquid crystal element is used as the displayelement, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode,a fringe field switching (FFS) mode, an axially symmetric alignedmicro-cell (ASM) mode, an optical compensated birefringence (OCB) mode,a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquidcrystal (AFLC) mode, or the like can be used.

Further, a normally black liquid crystal display device such as atransmissive liquid crystal display device utilizing a verticalalignment (VA) mode may also be used. There are some examples of avertical alignment mode; for example, a multi-domain vertical alignment(MVA) mode, a patterned vertical alignment (PVA) mode, an ASV mode, orthe like can be employed.

<Display Device Using Light-Emitting Element as Display Element>

The display device 700 illustrated in FIG. 31 includes a light-emittingelement 782. The light-emitting element 782 includes a conductive film784, an EL layer 786, and a conductive film 788. The display device 700shown in FIG. 31 is capable of displaying an image by light emissionfrom the EL layer 786 included in the light-emitting element 782.

The conductive film 784 is connected to the conductive films functioningas a source electrode and a drain electrode included in the transistor750. The conductive film 784 is formed over the planarization insulatingfilm 770 to function as a pixel electrode, i.e., one electrode of thedisplay element. A conductive film which transmits visible light or aconductive film which reflects visible light can be used for theconductive film 784. The conductive film which transmits visible lightcan be formed using a material including one kind selected from indium(In), zinc (Zn), and tin (Sn), for example. The conductive film whichreflects visible light can be formed using a material including aluminumor silver, for example.

In the display device 700 shown in FIG. 31, an insulating film 730 isprovided over the planarization insulating film 770 and the conductivefilm 784. The insulating film 730 covers part of the conductive film784. Note that the light-emitting element 782 has a top emissionstructure. Therefore, the conductive film 788 has a light-transmittingproperty and transmits light emitted from the EL layer 786. Although thetop-emission structure is described as an example in this embodiment,one embodiment of the present invention is not limited thereto. Abottom-emission structure in which light is emitted to the conductivefilm 784 side, or a dual-emission structure in which light is emitted toboth the conductive film 784 side and the conductive film 788 side maybe employed.

The coloring film 736 is provided to overlap with the light-emittingelement 782, and the light-blocking film 738 is provided to overlap withthe insulating film 730 and to be included in the lead wiring portion711 and in the source driver circuit portion 704. The coloring film 736and the light-blocking film 738 are covered with the insulating film734. A space between the light-emitting element 782 and the insulatingfilm 734 is filled with the sealing film 732. Although a structure withthe coloring film 736 is described as the display device 700 shown inFIG. 31, the structure is not limited thereto. In the case where the ELlayer 786 is formed by a separate coloring method, the coloring film 736is not necessarily provided.

The structure described in this embodiment can be used in appropriatecombination with any of the structures described in the otherembodiments.

Embodiment 5

In this embodiment, a display device that includes a semiconductordevice of one embodiment of the present invention is described withreference to FIGS. 32A to 32C.

The display device illustrated in FIG. 32A includes a region includingpixels of display elements (hereinafter the region is referred to as apixel portion 502), a circuit portion being provided outside the pixelportion 502 and including a circuit for driving the pixels (hereinafterthe portion is referred to as a driver circuit portion 504), circuitseach having a function of protecting an element (hereinafter thecircuits are referred to as protection circuits 506), and a terminalportion 507. Note that the protection circuits 506 are not necessarilyprovided.

A part or the whole of the driver circuit portion 504 is preferablyformed over a substrate over which the pixel portion 502 is formed, inwhich case the number of components and the number of terminals can bereduced. When a part or the whole of the driver circuit portion 504 isnot formed over the substrate over which the pixel portion 502 isformed, the part or the whole of the driver circuit portion 504 can bemounted by COG or tape automated bonding (TAB).

The pixel portion 502 includes a plurality of circuits for drivingdisplay elements arranged in X rows (X is a natural number of 2 or more)and Y columns (Y is a natural number of 2 or more) (hereinafter, suchcircuits are referred to as pixel circuits 501). The driver circuitportion 504 includes driver circuits such as a circuit for supplying asignal (scan signal) to select a pixel (hereinafter, the circuit isreferred to as a gate driver 504 a) and a circuit for supplying a signal(data signal) to drive a display element in a pixel (hereinafter, thecircuit is referred to as a source driver 504 b).

The gate driver 504 a includes a shift register or the like. The gatedriver 504 a receives a signal for driving the shift register throughthe terminal portion 507 and outputs a signal. For example, the gatedriver 504 a receives a start pulse signal, a clock signal, or the likeand outputs a pulse signal. The gate driver 504 a has a function ofcontrolling the potentials of wirings supplied with scan signals(hereinafter, such wirings are referred to as scan lines GL_1 to GL_X).Note that a plurality of gate drivers 504 a may be provided to controlthe scan lines GL_1 to GL_X separately. Alternatively, the gate driver504 a has a function of supplying an initialization signal. Withoutbeing limited thereto, the gate driver 504 a can supply another signal.

The source driver 504 b includes a shift register or the like. Thesource driver 504 b receives a signal (video signal) from which a datasignal is derived, as well as a signal for driving the shift register,through the terminal portion 507. The source driver 504 b has a functionof generating a data signal to be written to the pixel circuit 501 whichis based on the video signal. In addition, the source driver 504 b has afunction of controlling output of a data signal in response to a pulsesignal produced by input of a start pulse signal, a clock signal, or thelike. Further, the source driver 504 b has a function of controlling thepotentials of wirings supplied with data signals (hereinafter suchwirings are referred to as data lines DL_1 to DL_Y). Alternatively, thesource driver 504 b has a function of supplying an initializationsignal. Without being limited thereto, the source driver 504 b cansupply another signal.

The source driver 504 b includes a plurality of analog switches or thelike, for example. The source driver 504 b can output, as the datasignals, signals obtained by time-dividing the video signal bysequentially turning on the plurality of analog switches. The sourcedriver 504 b may include a shift register or the like.

A pulse signal and a data signal are input to each of the plurality ofpixel circuits 501 through one of the plurality of scan lines GLsupplied with scan signals and one of the plurality of data lines DLsupplied with data signals, respectively. Writing and holding of thedata signal to and in each of the plurality of pixel circuits 501 arecontrolled by the gate driver 504 a. For example, to the pixel circuit501 in the m-th row and the n-th column (m is a natural number of lessthan or equal to X, and n is a natural number of less than or equal toY), a pulse signal is input from the gate driver 504 a through the scanline GL_m, and a data signal is input from the source driver 504 bthrough the data line DL_n in accordance with the potential of the scanline GL_m.

The protection circuit 506 shown in FIG. 32A is connected to, forexample, the scan line GL between the gate driver 504 a and the pixelcircuit 501. Alternatively, the protection circuit 506 is connected tothe data line DL between the source driver 504 b and the pixel circuit501. Alternatively, the protection circuit 506 can be connected to awiring between the gate driver 504 a and the terminal portion 507.Alternatively, the protection circuit 506 can be connected to a wiringbetween the source driver 504 b and the terminal portion 507. Note thatthe terminal portion 507 means a portion having terminals for inputtingpower, control signals, and video signals to the display device fromexternal circuits.

The protection circuit 506 is a circuit that electrically connects awiring connected to the protection circuit to another wiring when apotential out of a certain range is applied to the wiring connected tothe protection circuit.

As illustrated in FIG. 32A, the protection circuits 506 are provided forthe pixel portion 502 and the driver circuit portion 504, so that theresistance of the display device to overcurrent generated byelectrostatic discharge (ESD) or the like can be improved. Note that theconfiguration of the protection circuits 506 is not limited to that, andfor example, the protection circuit 506 may be configured to beconnected to the gate driver 504 a or the protection circuit 506 may beconfigured to be connected to the source driver 504 b. Alternatively,the protection circuit 506 may be configured to be connected to theterminal portion 507.

In FIG. 32A, an example in which the driver circuit portion 504 includesthe gate driver 504 a and the source driver 504 b is shown; however, thestructure is not limited thereto. For example, only the gate driver 504a may be formed and a separately prepared substrate where a sourcedriver circuit is formed (e.g., a driver circuit substrate formed with asingle crystal semiconductor film or a polycrystalline semiconductorfilm) may be mounted.

Each of the plurality of pixel circuits 501 in FIG. 32A can have thestructure illustrated in FIG. 32B, for example.

The pixel circuit 501 illustrated in FIG. 32B includes a liquid crystalelement 570, a transistor 550, and a capacitor 560. As the transistor550, any of the transistors described in the above embodiment, forexample, can be used.

The potential of one of a pair of electrodes of the liquid crystalelement 570 is set in accordance with the specifications of the pixelcircuit 501 as appropriate. The alignment state of the liquid crystalelement 570 depends on written data. A common potential may be suppliedto one of the pair of electrodes of the liquid crystal element 570included in each of the plurality of pixel circuits 501. Further, thepotential supplied to one of the pair of electrodes of the liquidcrystal element 570 in the pixel circuit 501 in one row may be differentfrom the potential supplied to one of the pair of electrodes of theliquid crystal element 570 in the pixel circuit 501 in another row.

As examples of a driving method of the display device including theliquid crystal element 570, any of the following modes can be given: aTN mode, an STN mode, a VA mode, an axially symmetric aligned micro-cell(ASM) mode, an optically compensated birefringence (OCB) mode, aferroelectric liquid crystal (FLC) mode, an antiferroelectric liquidcrystal (AFLC) mode, an MVA mode, a patterned vertical alignment (PVA)mode, an IPS mode, an FFS mode, a transverse bend alignment (TBA) mode,and the like. Other examples of the driving method of the display deviceinclude an electrically controlled birefringence (ECB) mode, a polymerdispersed liquid crystal (PDLC) mode, a polymer network liquid crystal(PNLC) mode, and a guest-host mode. Note that the present invention isnot limited to these examples, and various liquid crystal elements anddriving methods can be applied to the liquid crystal element and thedriving method thereof.

In the pixel circuit 501 in the m-th row and the n-th column, one of asource electrode and a drain electrode of the transistor 550 iselectrically connected to the data line DL_n, and the other iselectrically connected to the other of the pair of electrodes of theliquid crystal element 570. A gate electrode of the transistor 550 iselectrically connected to the scan line GL_m. The transistor 550 has afunction of controlling whether to write a data signal by being turnedon or off.

One of a pair of electrodes of the capacitor 560 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a potential supply line VL), and the other iselectrically connected to the other of the pair of electrodes of theliquid crystal element 570. The potential of the potential supply lineVL is set in accordance with the specifications of the pixel circuit 501as appropriate. The capacitor 560 functions as a storage capacitor forstoring written data.

For example, in the display device including the pixel circuit 501 inFIG. 32B, the pixel circuits 501 are sequentially selected row by row bythe gate driver 504 a illustrated in FIG. 32A, whereby the transistors550 are turned on and a data signal is written.

When the transistors 550 are turned off, the pixel circuits 501 in whichthe data has been written are brought into a holding state. Thisoperation is sequentially performed row by row; thus, an image can bedisplayed.

Alternatively, each of the plurality of pixel circuits 501 in FIG. 32Acan have the structure illustrated in FIG. 32C, for example.

The pixel circuit 501 illustrated in FIG. 32C includes transistors 552and 554, a capacitor 562, and a light-emitting element 572. Any of thetransistors described in the above embodiment, for example, can be usedas one or both of the transistors 552 and 554.

One of a source electrode and a drain electrode of the transistor 552 iselectrically connected to a wiring to which a data signal is supplied(hereinafter referred to as a signal line DL_n). A gate electrode of thetransistor 552 is electrically connected to a wiring to which a gatesignal is supplied (hereinafter referred to as a scan line GL_m).

The transistor 552 has a function of controlling whether to write a datasignal by being turned on or off.

One of a pair of electrodes of the capacitor 562 is electricallyconnected to a wiring to which a potential is supplied (hereinafterreferred to as a potential supply line VL_a), and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 552.

The capacitor 562 functions as a storage capacitor for storing writtendata.

One of a source electrode and a drain electrode of the transistor 554 iselectrically connected to the potential supply line VL_a. Further, agate electrode of the transistor 554 is electrically connected to theother of the source electrode and the drain electrode of the transistor552.

One of an anode and a cathode of the light-emitting element 572 iselectrically connected to a potential supply line VL_b, and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 554.

As the light-emitting element 572, an organic electroluminescent element(also referred to as an organic EL element) or the like can be used, forexample. Note that the light-emitting element 572 is not limited to anorganic EL element; an inorganic EL element including an inorganicmaterial may be used.

A high power supply potential VDD is supplied to one of the potentialsupply line VL_a and the potential supply line VL_b, and a low powersupply potential VSS is supplied to the other.

For example, in the display device including the pixel circuit 501 inFIG. 32C, the pixel circuits 501 are sequentially selected row by row bythe gate driver 504 a illustrated in FIG. 32A, whereby the transistors552 are turned on and a data signal is written.

When the transistors 552 are turned off, the pixel circuits 501 in whichthe data has been written are brought into a holding state. Further, theamount of current flowing between the source electrode and the drainelectrode of the transistor 554 is controlled in accordance with thepotential of the written data signal. The light-emitting element 572emits light with a luminance corresponding to the amount of flowingcurrent. This operation is sequentially performed row by row; thus, animage can be displayed.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 6

In this embodiment, a display module and electronic appliances thatinclude a semiconductor device of one embodiment of the presentinvention are described with reference to FIG. 33 and FIGS. 34A to 34H.

In a display module 8000 illustrated in FIG. 33, a touch panel 8004connected to an FPC 8003, a display panel 8006 connected to an FPC 8005,a backlight unit 8007, a frame 8009, a printed board 8010, and a battery8011 are provided between an upper cover 8001 and a lower cover 8002.

The semiconductor device of one embodiment of the present invention canbe used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and can be formed to overlap the display panel 8006. Acounter substrate (sealing substrate) of the display panel 8006 can havea touch panel function. A photosensor may be provided in each pixel ofthe display panel 8006 to form an optical touch panel.

The backlight unit 8007 includes a light source 8008. Note that althougha structure in which the light sources 8008 are provided over thebacklight unit 8007 is illustrated in FIG. 33, one embodiment of thepresent invention is not limited to this structure. For example, astructure in which the light source 8008 is provided at an end portionof the backlight unit 8007 and a light diffusion plate is furtherprovided may be employed. Note that the backlight unit 8007 need not beprovided in the case where a self-luminous light-emitting element suchas an organic EL element is used or in the case where a reflective panelor the like is employed.

The frame 8009 protects the display panel 8006 and also functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 may function asa radiator plate.

The printed board 8010 is provided with a power supply circuit and asignal processing circuit for outputting a video signal and a clocksignal. As a power source for supplying power to the power supplycircuit, an external commercial power source or a power source using thebattery 8011 provided separately may be used. The battery 8011 can beomitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

FIGS. 34A to 34H illustrate electronic appliances. These electronicappliances can include a housing 9000, a display portion 9001, a speaker9003, an LED lamp 9004, operation keys 9005 (including a power switch oran operation switch), a connection terminal 9006, a sensor 9007 (asensor having a function of measuring or sensing force, displacement,position, speed, acceleration, angular velocity, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, current, voltage, electric power,radiation, flow rate, humidity, gradient, oscillation, odor, or infraredray), a microphone 9008, and the like.

FIG. 34A illustrates a mobile computer that can include a switch 9009,an infrared port 9010, and the like in addition to the above components.FIG. 34B illustrates a portable image reproducing device (e.g., a DVDplayer) that is provided with a memory medium and can include a seconddisplay portion 9002, a memory medium reading portion 9011, and the likein addition to the above components. FIG. 34C illustrates a goggle-typedisplay that can include the second display portion 9002, a support9012, an earphone 9013, and the like in addition to the abovecomponents. FIG. 34D illustrates a portable game machine that caninclude the memory medium reading portion 9011 and the like in additionto the above components. FIG. 34E illustrates a digital camera that hasa television reception function and can include an antenna 9014, ashutter button 9015, an image receiving portion 9016, and the like inaddition to the above components. FIG. 34F illustrates a portable gamemachine that can include the second display portion 9002, the memorymedium reading portion 9011, and the like in addition to the abovecomponents. FIG. 34G illustrates a television receiver that can includea tuner, an image processing portion, and the like in addition to theabove components. FIG. 34H illustrates a portable television receiverthat can include a charger 9017 capable of transmitting and receivingsignals, and the like in addition to the above components.

The electronic appliances illustrated in FIGS. 34A to 34H can have avariety of functions, for example, a function of displaying a variety ofdata (a still image, a moving image, a text image, and the like) on thedisplay portion, a touch panel function, a function of displaying acalendar, date, time, and the like, a function of controlling a processwith a variety of software (programs), a wireless communicationfunction, a function of being connected to a variety of computernetworks with a wireless communication function, a function oftransmitting and receiving a variety of data with a wirelesscommunication function, a function of reading a program or data storedin a memory medium and displaying the program or data on the displayportion, and the like. Furthermore, the electronic appliance including aplurality of display portions can have a function of displaying imagedata mainly on one display portion while displaying text data on anotherdisplay portion, a function of displaying a three-dimensional image bydisplaying images on a plurality of display portions with a parallaxtaken into account, or the like. Furthermore, the electronic applianceincluding an image receiving portion can have a function of shooting astill image, a function of taking a moving image, a function ofautomatically or manually correcting a shot image, a function of storinga shot image in a memory medium (an external memory medium or a memorymedium incorporated in the camera), a function of displaying a shotimage on the display portion, or the like. Note that functions that canbe provided for the electronic appliances illustrated in FIGS. 34A to34H are not limited to those described above, and the electronicappliances can have a variety of functions.

The electronic appliances described in this embodiment each include thedisplay portion for displaying some sort of data. Note that thesemiconductor device of one embodiment of the present invention can alsobe used for an electronic appliance that does not have a displayportion.

The electronic appliances described in this embodiment each include thedisplay portion for displaying some sort of data. Note that thesemiconductor device of one embodiment of the present invention can alsobe used for an electronic appliance that does not have a displayportion.

Example 1

In this example, the amount of oxygen released from an insulating filmincluded in a semiconductor device of one embodiment of the presentinvention was measured. Samples 1 to 10 described below were used forevaluation in this example.

(Sample 1)

Sample 1 was formed in such a manner that a 100-nm-thick silicon oxidefilm was formed over a glass substrate with a sputtering apparatus. Thesilicon oxide film was deposited under the conditions where thesubstrate temperature was 100° C., an oxygen gas at a flow rate of 50sccm was introduced into a chamber, the pressure was 0.5 Pa, and a DCpower of 6000 W was supplied to a silicon sputtering target.

(Sample 2)

Sample 2 was formed in such a manner that a 100-nm-thick silicon nitridefilm and a 400-nm-thick silicon oxynitride film over the 100-nm-thicksilicon nitride film were formed over a glass substrate with a PECVDapparatus and subjected to heat treatment.

(Sample 3)

Sample 3 was formed in such a manner that a 100-nm-thick silicon nitridefilm and a 400-nm-thick silicon oxynitride film over the 100-nm-thicksilicon nitride film were formed over a glass substrate with a PECVDapparatus and subjected to heat treatment. Then, oxygen additiontreatment is performed on the silicon oxynitride film.

(Sample 4)

Sample 4 was formed in such a manner that a 100-nm-thick silicon nitridefilm and a 400-nm-thick silicon oxynitride film over the 100-nm-thicksilicon nitride film were formed over a glass substrate with a PECVDapparatus and subjected to heat treatment. Next, a 5-nm-thick oxidesemiconductor film (an IGZO film with In:Ga:Zn=1:1:1) was formed with asputtering apparatus. Then, oxygen addition treatment was performedthrough the oxide semiconductor film. After that, the oxidesemiconductor film was removed to expose the silicon oxynitride film.

(Sample 5)

Sample 5 was formed in such a manner that a 100-nm-thick silicon nitridefilm and a 400-nm-thick silicon oxynitride film over the 100-nm-thicksilicon nitride film were formed over a glass substrate with a PECVDapparatus and subjected to heat treatment. Next, a 5-nm-thick tungstenfilm was formed with a sputtering apparatus. Then, oxygen additiontreatment was performed through the tungsten film. After that, thetungsten film was removed to expose the silicon oxynitride film.

(Sample 6)

Sample 6 was formed in such a manner that a 100-nm-thick silicon nitridefilm and a 400-nm-thick silicon oxynitride film over the 100-nm-thicksilicon nitride film were formed over a glass substrate with a PECVDapparatus and subjected to heat treatment. Next, a 5-nm-thick tantalumnitride film was formed with a sputtering apparatus. Then, oxygenaddition treatment was performed through the tantalum nitride film.After that, the tantalum nitride film was removed to expose the siliconoxynitride film.

(Sample 7)

Sample 7 was formed in such a manner that a 100-nm-thick silicon nitridefilm and a 400-nm-thick silicon oxynitride film over the 100-nm-thicksilicon nitride film were formed over a glass substrate with a PECVDapparatus and subjected to heat treatment. Next, a 5-nm-thick titaniumfilm was formed with a sputtering apparatus. Then, oxygen additiontreatment was performed through the titanium film. After that, thetitanium film was removed to expose the silicon oxynitride film.

(Sample 8)

Sample 8 was formed in such a manner that a 100-nm-thick silicon nitridefilm and a 400-nm-thick silicon oxynitride film over the 100-nm-thicksilicon nitride film were formed over a glass substrate with a PECVDapparatus and subjected to heat treatment. Next, a 5-nm-thick aluminumfilm was formed with a sputtering apparatus. Then, oxygen additiontreatment was performed through the aluminum film. After that, thealuminum film was removed to expose the silicon oxynitride film.

(Sample 9)

Sample 9 was formed in such a manner that a 100-nm-thick silicon nitridefilm and a 400-nm-thick silicon oxynitride film over the 100-nm-thicksilicon nitride film were formed over a glass substrate with a PECVDapparatus and subjected to heat treatment. Next, a 5-nm-thick ITSO filmwas formed with a sputtering apparatus. Then, oxygen addition treatmentwas performed through the ITSO film. After that, the ITSO film wasremoved to expose the silicon oxynitride film. Note that the compositionratio of In₂O₃ to SnO₂ and SiO₂ in the target used for forming the ITSOfilm was 85:10:5 [wt %].

(Sample 10)

Sample 10 was formed in such a manner that a 100-nm-thick siliconnitride film was formed over a glass substrate with a PECVD apparatus.

The heat treatment performed on each of Samples 2 to 9 was performed at650° C. for 6 minutes in a nitrogen atmosphere with an RTA apparatus. Bythe heat treatment, oxygen included in the silicon oxynitride film atthe time of deposition is released from the silicon oxynitride film.

The silicon nitride film used in each of Samples 2 to 10 was depositedunder the conditions where the substrate temperature was 350° C.; asilane gas at a flow rate of 200 sccm, a nitrogen gas at a flow rate of2000 sccm, and an ammonia gas at a flow rate of 2000 sccm wereintroduced into a chamber; the pressure was 100 Pa; and an RF power of2000 W was supplied between parallel-plate electrodes provided in aPECVD apparatus.

The silicon oxynitride film in each of Samples 2 to 9 was depositedunder the conditions where the substrate temperature was 220° C., asilane gas at a flow rate of 160 sccm and a dinitrogen monoxide gas at aflow rate of 4000 sccm were introduced into a chamber, the pressure was200 Pa, and an RF power of 1500 W was supplied between parallel-plateelectrodes provided in a PECVD apparatus.

The oxygen addition treatment performed on each of Samples 3 to 9 wasconducted with an etching apparatus under the conditions where thesubstrate temperature was 40° C., an oxygen gas (¹⁶O) at a flow rate of250 sccm was introduced into a chamber, the pressure was 15 Pa, and anRF power of 4500 W was supplied between parallel-plate electrodesprovided in the etching apparatus so that a bias would be applied to thesubstrate side.

The amount of a gas having a mass-to-charge ratio (M/z) of 32, i.e.,oxygen (O₂), released from each of Samples 1 to 10 was measured. A TDSanalysis apparatus was used for measuring the amount of released gas.

FIG. 35 shows the TDS measurement results of Samples 1 to 10. In FIG.35, the horizontal axis shows the name of the sample, and the verticalaxis represents the amount of released gas with M/z=32.

According to the results in FIG. 35, the amount of a gas with M/z=32released from Sample 1 was 5×10²⁰/cm³. The amount of a gas with M/z=32released from Sample 2 was 3×10¹⁸/cm³. The amount of a gas with M/z=32released from Sample 3 was 2×10¹⁹/cm³. The amount of a gas with M/z=32released from Sample 4 was 3×10²⁰/cm³. The amount of a gas with M/z=32released from Sample 5 was 5×10¹⁹/cm³. The amount of a gas with M/z=32released from Sample 6 was 2×10²¹/cm³. The amount of a gas with M/z=32released from Sample 7 was 1×10²¹/cm³. The amount of a gas with M/z=32released from Sample 8 was 5×10²⁰/cm³. The amount of a gas with M/z=32released from Sample 9 was 8×10²⁰/cm³. The amount of a gas with M/z=32released from Sample 10 was 3×10¹⁸/cm³.

The results indicate that such an amount of oxygen was released fromSample 1 because a sputtering apparatus was used for depositing thesilicon oxide film in Sample 1 and thus the silicon oxide film hadexcess oxygen, and further, heat treatment was not performed. Inaddition, the amount of oxygen released from Sample 2 was smaller thanthose of oxygen released from the other samples because heat treatmentwas performed after formation of the silicon oxynitride film in Sample 2to release oxygen from the silicon oxynitride film. Furthermore, theamount of oxygen released from each of Samples 3 to 9 was larger thanthat of oxygen released from Sample 2 because oxygen addition treatmentwas performed on each of Samples 3 to 9 after heat treatment. Moreover,the amount of oxygen released from each of Samples 4 to 9 was largerthan that of oxygen released from Sample 3 because the metal film, themetal nitride film, or the metal oxide film was provided over thesilicon oxynitride film, and oxygen was added to the silicon oxynitridefilm through the metal film, the metal nitride film, or the metal oxidefilm. In particular, the amount of oxygen released from Sample 6, i.e.,the structure that included a silicon oxynitride film and a tantalumnitride film over the silicon oxynitride film and was subjected tooxygen addition treatment, was the largest of those of oxygen releasedfrom the samples. Furthermore, the amount of oxygen released from Sample10 was small because a gas containing oxygen was not used fordeposition.

As described above, it is proved that Sample 1 and Samples 3 to 9 areeach capable of releasing oxygen by heating, and the amount of oxygenreleased from each of Sample 1 and Samples 3 to 9 was greater than orequal to 1×10¹⁹/cm³, which is estimated as oxygen molecules. Therefore,these samples can be used as the third insulating film of thesemiconductor device of one embodiment of the present invention.Furthermore, Sample 2 and Sample 10 can each be used as the fourthinsulating film.

The structure described above in this example can be combined with anyof the structures described in the other embodiments and examples asappropriate.

Example 2

In this example, the oxygen concentration of an insulating film includedin the semiconductor device of one embodiment of the present invention,here, a silicon oxynitride film, was measured. Sample A1 and Sample A2described below were formed and used for evaluation in this example.

(Sample A1)

Sample A1 was formed as follows. A 100-nm-thick silicon nitride film wasformed over a glass substrate, a 400-nm-thick silicon oxynitride filmwas formed over the silicon nitride film, and heat treatment wasperformed. Then, a 5-nm-thick tantalum nitride film was formed with asputtering apparatus. After that, the tantalum nitride film was removedto expose the silicon oxynitride film. Note that Sample A1 is forcomparison.

(Sample A2)

Sample A2 was formed as follows. A 100-nm-thick silicon nitride film wasformed over a glass substrate, a 400-nm-thick silicon oxynitride filmwas formed over the silicon nitride film, and heat treatment wasperformed. Then, a 5-nm-thick tantalum nitride film was formed with asputtering apparatus, and oxygen addition treatment was performed. Afterthat, the tantalum nitride film was removed to expose the siliconoxynitride film.

The deposition conditions of the silicon nitride film in each of SamplesA1 and A2 were the same as those described in Example 1. The heattreatment conditions for each of Samples A1 and A2 were the same asthose described in Example 1.

The oxygen addition treatment performed on Sample A2 was conducted withan etching apparatus under the conditions where the substratetemperature was 40° C., an oxygen gas (¹⁶O) at a flow rate of 150 sccmand an oxygen gas (¹⁸O) at a flow rate of 100 sccm were introduced intoa chamber, the pressure was 15 Pa, and an RF power of 4500 W wassupplied between parallel-plate electrodes provided in the etchingapparatus so that a bias would be applied to the substrate side. Sincethe silicon oxynitride film included oxygen (¹⁶O) at a main componentlevel when deposited, an oxygen gas (¹⁸O) was used to exactly measurethe amount of oxygen added by the oxygen addition treatment.

The oxygen concentrations of Samples A1 and A2 were measured. A SIMSanalysis apparatus was used for measuring the oxygen concentration, andoxygen to be measured was ¹⁸O.

FIGS. 36A and 36B show SIMS measurement results of Sample A1 and SampleA2, respectively.

In FIGS. 36A and 36B, the vertical axis and the horizontal axisrepresent ¹⁸O concentration and depth, respectively. A dashed line inFIGS. 36A and 36B denotes the vicinity of the interface between thesilicon oxynitride film and the silicon nitride film. Furthermore, inFIGS. 36A and 36B, “SiON” denotes the silicon oxynitride film, and “SiN”denotes the silicon nitride film.

Since Sample A1 for comparison was not subjected to oxygen additiontreatment, the silicon oxynitride film includes oxygen (¹⁸O) atapproximately 1.0×10²⁰ atoms/cm³ as shown in FIG. 36A. This issubstantially equal to the natural abundance of oxygen (¹⁸O) (0.2%),which means that the silicon oxynitride film of Sample A1 hardlyincludes oxygen (¹⁸O). Meanwhile, the silicon oxynitride film includedin the semiconductor device of one embodiment of the present inventionincludes oxygen (¹⁸O) at higher than or equal to 8.0×10²⁰ atoms/cm³ andlower than or equal to 1×10²² atoms/cm³ as shown in FIG. 36B. Thus, theoxygen concentration of the silicon oxynitride film can be increased byoxygen addition treatment. In addition, it was found that the oxygenintroduced into the silicon oxynitride film by the oxygen additiontreatment was substantially uniformly included in the silicon oxynitridefilm.

The structure described above in this example can be combined with anyof the structures described in the other embodiments as appropriate.

Example 3

In this example, a transistor corresponding to the transistor 170 inFIGS. 7A to 7C was formed and tests for electrical characteristics andreliability were performed. In this example, Sample B1, Sample B2,Sample C1, and Sample C2 were formed and used for evaluation. Note thatSamples B1 and B2 are transistors for comparison, and Samples C1 and C2are transistors of one embodiment of the present invention. To form eachof Samples B1, B2, C1, and C2, 20 transistors were formed over asubstrate.

The samples formed in this example are described below. Note that thereference numerals used for the transistor 170 in FIGS. 7A to 7C areused in the following description.

(Sample B1 and Sample B2)

Sample B1 included 20 transistors each having a channel length L of 2 μmand a channel width W of 50 μm, and Sample B2 included 20 transistorseach having a channel length L of 6 μm and a channel width W of 50 μm.Thus, both the samples had the same structure by the same method, exceptfor the channel length L.

First, the conductive film 104 was formed over the substrate 102. As thesubstrate 102, a glass substrate was used. Furthermore, as theconductive film 104, a 100-nm-thick tungsten film was formed with asputtering apparatus.

Next, the insulating films 106 and 107 were formed over the substrate102 and the conductive film 104. As the insulating film 106, a400-nm-thick silicon nitride film was formed with a PECVD apparatus. Asthe insulating film 107, a 50-nm-thick silicon oxynitride film wasformed with a PECVD apparatus.

Then, the oxide semiconductor film 108 was formed over the insulatingfilm 107. As the oxide semiconductor film 108, a 35-nm-thick IGZO filmwas formed with a sputtering apparatus. Note that the oxidesemiconductor film 108 was deposited under the conditions where thesubstrate temperature was 170° C., an argon gas at a flow rate of 100sccm and an oxygen gas at a flow rate of 100 sccm were introduced into achamber, the pressure was 0.6 Pa, and an AC power of 2500 W was appliedto a metal oxide sputtering target (In:Ga:Zn=1:1:1).

Then, first heat treatment was performed. As the first heat treatment,heat treatment was performed at 450° C. for 1 hour in a nitrogenatmosphere and then heat treatment was performed at 450° C. for 1 hourin a mixed atmosphere of nitrogen and oxygen.

Next, the conductive films 112 a and 112 b were formed over theinsulating film 107 and the oxide semiconductor film 108. As theconductive films 112 a and 112 b, a 50-nm-thick tungsten film, a400-nm-thick aluminum film, and a 100-nm-thick titanium film weresuccessively formed in vacuum with a sputtering apparatus.

After that, the insulating film 114 and the insulating film 116 wereformed over the insulating film 107, the oxide semiconductor film 108,and the conductive films 112 a and 112 b. As the insulating film 114, a50-nm-thick silicon oxynitride film was formed with a PECVD apparatus.As the insulating film 116, a 400-nm-thick silicon oxynitride film wasformed with a PECVD apparatus. Note that the insulating film 114 and theinsulating film 116 were formed successively in vacuum with a PECVDapparatus.

The insulating film 114 was deposited under the conditions where thesubstrate temperature was 220° C., a silane gas at a flow rate of 50sccm and a dinitrogen monoxide gas at a flow rate of 2000 sccm wereintroduced into a chamber, the pressure was 20 Pa, and an RF power of100 W was supplied between parallel-plate electrodes provided in a PECVDapparatus. The insulating film 116 was deposited under the conditionswhere the substrate temperature was 220° C., a silane gas at a flow rateof 160 sccm and a dinitrogen monoxide gas at a flow rate of 4000 sccmwere introduced into a chamber, the pressure was 200 Pa, and an RF powerof 1500 W was supplied between parallel-plate electrodes provided in aPECVD apparatus.

Then, second heat treatment was performed. The second heat treatment wasperformed at 350° C. for 1 hour in a mixed gas atmosphere of nitrogenand oxygen.

Next, the insulating film 118 was formed over the insulating film 116.As the insulating film 118, a 100-nm-thick silicon nitride film wasformed with a PECVD apparatus. The insulating film 118 was depositedunder the conditions where the substrate temperature was 350° C., asilane gas at a flow rate of 50 sccm, a nitrogen gas at a flow rate of5000 sccm, and an ammonia gas at a flow rate of 100 sccm were introducedinto a chamber, the pressure was 100 Pa, and an RF power of 1000 W wassupplied between parallel-plate electrodes provided in a PECVDapparatus.

Next, the opening 142 c reaching the conductive film 112 b and theopenings 142 a and 142 b reaching the conductive film 104 were formed.The openings 142 a, 142 b, and 142 c were formed with a dry etchingapparatus.

Next, a conductive film was formed over the insulating film 118 to coverthe openings 142 a, 142 b, and 142 c and processed to form theconductive films 120 a and 120 b. For the conductive films 120 a and 120b, a 100-nm-thick ITSO film was formed with a sputtering apparatus. Thecomposition of a target used for forming the ITSO film was the same asthat described in Example 1.

Then, third heat treatment was performed. The third heat treatment wasperformed at 250° C. for 1 hour in a nitrogen atmosphere.

Through the above process, Samples B1 and Sample B2 were formed.

(Sample C1 and Sample C2)

Sample C1 included 20 transistors each having a channel length L of 2 μmand a channel width W of 50 μm, and Sample C2 included 20 transistorseach having a channel length L of 6 μm and a channel width W of 50 μm.Thus, both the samples had the same structure except for the channellength L and were formed by the same formation method.

The process for forming Samples C1 and C2 are different from that forforming Samples B1 and B2 described above in the following steps. Thesteps other than the following steps are the same as those for SamplesB1 and B2.

After the second heat treatment, the film 130 capable of inhibitingrelease of oxygen was formed over the insulating film 116. As the film130, a 5-nm-thick tantalum oxide film was formed with a sputteringapparatus.

Next, oxygen addition treatment was performed on the oxide semiconductorfilm 108 and the insulating films 114 and 116 through the film 130. Thefilm 130 became the insulating film 131 owing to the oxygen additiontreatment. As the insulating film 131, a tantalum oxide film was formed.The conditions of the oxygen addition treatment were the same as thosedescribed in Example 1.

Next, the insulating film 118 was formed over the insulating film 131.As the insulating film 118, a 100-nm-thick silicon nitride film wasformed with a PECVD apparatus. In this manner, in Samples C1 and C2 ofthis example, the insulating film 131 was not removed. That is, SamplesC1 and C2 each have a structure of the transistor 170 in FIGS. 7A to 7Cin which the insulating film 131 is provided between the insulating film116 and the insulating film 118.

Through the above processes, Samples B1, B2, C1, and C2 were formed.

FIGS. 37A and 37B and FIGS. 38A and 38B show electrical characteristicsof Samples B1, B2, C1, and C2.

Note that FIG. 37A shows electrical characteristics of Sample B1, FIG.37B shows electrical characteristics of Sample B2, FIG. 38A shows theelectrical characteristics of Sample C1, and FIG. 38B shows electricalcharacteristics of Sample C2. In FIGS. 37A and 37B and FIGS. 38A and38B, the horizontal axis and the vertical axis represent gate voltage(VG) and drain current (ID), respectively, and data of the 20transistors are superimposed on each other. Furthermore, voltage betweenthe source electrode and the drain electrode (the voltage is expressedas VD) was set at 10 V, and VG was applied from −15 V to 20 V atintervals of 0.5 V.

The results in FIGS. 37A and 37B and FIGS. 38A and 38B show thatvariation among the transistors is large in Samples B1 and B2. Inparticular, variation in characteristics among the transistors having achannel length L of 2 μm in Sample B1 is large and the transistors havenormally-on characteristics. In contrast, variation in characteristicsamong the transistors is small in Samples C1 and C2. Furthermore,Samples C1 and C2 have favorable rising characteristics in the vicinityof 0 V.

Next, reliability tests were performed on Samples B1, C1, and C2. Forthe reliability tests, a bias-temperature stress test (hereinafter,referred to as gate bias temperature (GBT) test) was used.

Note that the GBT test is one kind of accelerated test and a change incharacteristics, caused by long-term usage, of transistors can beevaluated in a short time. In particular, the amount of shift inthreshold voltage (ΔVth) of the transistor between before and after aGBT test is an important indicator for examining reliability. Thesmaller the shift in the threshold voltage (ΔVth) between before andafter a GBT test is, the higher the reliability of the transistor is.

The GBT tests in this example were performed under the conditions wherethe gate voltage (VG) was ±30 V; the drain voltage (VD) and the sourcevoltage (VS) were 0 V (COMMON); the stress temperature was 60° C.; thetime for stress application was one hour; and two kinds of measurementenvironments, a dark environment and a photo environment (irradiationwith light having approximately 10000 lx with a white LED), wereemployed. In other words, the source electrode and the drain electrodeof the transistor were set at the same potential, and a potentialdifferent from that of the source and drain electrodes was applied tothe gate electrode for a certain time (one hour, here). A case where thepotential applied to the gate electrode is higher than that of thesource and drain electrodes is called positive stress, and a case wherethe potential applied to the gate electrode is lower than that of thesource and drain electrodes is called negative stress. Therefore, incombination with the measurement environments, the GBT stress test wasperformed under four stress conditions: dark positive stress, darknegative stress, photo positive stress, and photo negative stress.

FIG. 39 shows the GBT test results of Samples B1, C1, and C2. In FIG.39, the horizontal axis shows the name of the sample and the verticalaxis represents the amount of change in the threshold voltage (ΔVth) ofthe transistor.

The results in FIG. 39 show that the amount of change in the thresholdvoltage (ΔVth) in the GBT stress test is small in Samples C1 and C2 ofone embodiment of the present invention. In particular, under theconditions of the GBT stress test with light irradiation (photo positivestress and photo negative stress), the amount of change in the thresholdvoltage (ΔVth) in Samples C1 and C2 is smaller than that in Sample B1,which is a comparative example.

Accordingly, the transistors of Samples C1 and C2 of this example havesmall variation in electrical characteristics and high reliability.

The structure described above in this example can be combined with anyof the structures described in the other embodiments and examples asappropriate.

This application is based on Japanese Patent Application serial no.2014-039151 filed with Japan Patent Office on Feb. 28, 2014, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A semiconductor device comprising: a transistorcomprising: a gate electrode; a gate insulating film over the gateelectrode; an oxide semiconductor film over the gate insulating film;and a source electrode and a drain electrode on and in contact with theoxide semiconductor film; a first insulating film on and in contact withthe source electrode, the drain electrode, and the oxide semiconductorfilm; a second insulating film on and in contact with the firstinsulating film; and a third insulating film on and in contact with thesecond insulating film, wherein the gate insulating film includes afirst layer comprising silicon nitride and a second layer comprisingsilicon oxynitride on the first layer, wherein each of the firstinsulating film and the second insulating film comprises siliconoxynitride, wherein the third insulating film comprises silicon nitride,wherein a thickness of the second insulating film is larger than athickness of the first insulating film and a thickness of the thirdinsulating film, and wherein a region of the second insulating film hasa spin density corresponding to a signal that appears at g=2.001 lowerthan 1.5×10¹⁸ spins/cm³ by electron spin resonance measurement.
 2. Thesemiconductor device according to claim 1, wherein the oxidesemiconductor film is provided on and in contact with the second layerof the gate insulating film.
 3. The semiconductor device according toclaim 1, wherein the oxide semiconductor film comprises O, In, Zn, andM, and wherein the M is one selected from the group consisting of Ti,Ga, Y, Zr, La, Ce, Nd, and Hf.
 4. The semiconductor device according toclaim 1, wherein the oxide semiconductor film includes a crystal part,and wherein the crystal part includes a portion whose c-axis is aparallel to a normal vector of a surface where the oxide semiconductorfilm is formed.
 5. The semiconductor device according to claim 1,wherein the signal that appears at g=2.001 is due to dangling bonds ofsilicon in the second insulating film.
 6. The semiconductor deviceaccording to claim 1, wherein an amount of oxygen molecules releasedfrom each of the first insulating film and the second insulating film isgreater than or equal to 1×10¹⁹/cm³ when measured by thermal desorptionspectroscopy.
 7. The semiconductor device according to claim 1, whereinan amount of oxygen molecules released from the third insulating film isless than 1×10¹⁹/cm³ when measured by the thermal desorptionspectroscopy.
 8. The semiconductor device according to claim 1, whereinthe thickness of the third insulating film is larger than the thicknessof the first insulating film.
 9. A display device comprising thesemiconductor device according to claim
 1. 10. A display modulecomprising: the display device according to claim 9; and a touch sensor.11. An electronic appliance comprising: the semiconductor deviceaccording to claim 1; and at least one of an operation key and abattery.
 12. A semiconductor device comprising: a transistor comprising:a gate electrode; a first insulating film over the gate electrode; asecond insulating film on and in contact with the first insulating film;an oxide semiconductor film on and in contact with the second insulatingfilm; and a source electrode and a drain electrode on and in contactwith first regions of the oxide semiconductor film; a third insulatingfilm on and in contact with the source electrode, the drain electrode,and a second region of the oxide semiconductor film; a fourth insulatingfilm on and in contact with the third insulating film; and a fifthinsulating film on and in contact with the fourth insulating film,wherein each of the second insulating film, the third insulating film,and the fourth insulating film comprises silicon oxynitride, whereineach of the first insulating film and the fifth insulating filmcomprises silicon nitride, wherein a thickness of the fourth insulatingfilm is larger than a thickness of the third insulating film and athickness of the fifth insulating film, and wherein a region of thefourth insulating film has a spin density corresponding to a signal thatappears at g=2.001 lower than 1.5×10¹⁸ spins/cm³ by electron spinresonance measurement.
 13. The semiconductor device according to claim12, wherein the oxide semiconductor film comprises O, In, Zn, and M, andwherein the M is one selected from the group consisting of Ti, Ga, Y,Zr, La, Ce, Nd, and Hf.
 14. The semiconductor device according to claim12, wherein the oxide semiconductor film includes a crystal part, andwherein the crystal part includes a portion whose c-axis is a parallelto a normal vector of a surface where the oxide semiconductor film isformed.
 15. The semiconductor device according to claim 12, wherein thesignal that appears at g=2.001 is due to dangling bonds of silicon inthe fourth insulating film.
 16. The semiconductor device according toclaim 12, wherein an amount of oxygen molecules released from each ofthe third insulating film and the fourth insulating film is greater thanor equal to 1×10¹⁹/cm³ when measured by thermal desorption spectroscopy.17. The semiconductor device according to claim 12, wherein an amount ofoxygen molecules released from the fifth insulating film is less than1×10¹⁹/cm³ when measured by the thermal desorption spectroscopy.
 18. Thesemiconductor device according to claim 12, wherein the thickness of thefifth insulating film is larger than the thickness of the thirdinsulating film.
 19. A display device comprising the semiconductordevice according to claim
 12. 20. A display module comprising: thedisplay device according to claim 19; and a touch sensor.
 21. Anelectronic appliance comprising: the semiconductor device according toclaim 12; and at least one of an operation key and a battery.
 22. Asemiconductor device comprising: a transistor comprising: a gateelectrode; a gate insulating film over the gate electrode; an oxidesemiconductor film over the gate insulating film; and a source electrodeand a drain electrode on and in contact with the oxide semiconductorfilm; a first insulating film on and in contact with the sourceelectrode, the drain electrode, and the oxide semiconductor film; asecond insulating film on and in contact with the first insulating film;and a third insulating film on and in contact with the second insulatingfilm, wherein the gate insulating film includes a first layer comprisingsilicon nitride and a second layer comprising silicon oxynitride on thefirst layer, wherein each of the first insulating film and the secondinsulating film comprises silicon oxynitride, wherein the thirdinsulating film comprises silicon nitride, and wherein a thickness ofthe second insulating film is larger than a thickness of the firstinsulating film and a thickness of the third insulating film.
 23. Thesemiconductor device according to claim 22, wherein the oxidesemiconductor film is provided on and in contact with the second layerof the gate insulating film.
 24. The semiconductor device according toclaim 22, wherein the oxide semiconductor film comprises O, In, Zn, andM, and wherein the M is one selected from the group consisting of Ti,Ga, Y, Zr, La, Ce, Nd, and Hf.
 25. The semiconductor device according toclaim 22, wherein the oxide semiconductor film includes a crystal part,and wherein the crystal part includes a portion whose c-axis is aparallel to a normal vector of a surface where the oxide semiconductorfilm is formed.
 26. The semiconductor device according to claim 22,wherein the thickness of the third insulating film is larger than thethickness of the first insulating film.
 27. A display device comprisingthe semiconductor device according to claim
 22. 28. A display modulecomprising: the display device according to claim 27; and a touchsensor.
 29. An electronic appliance comprising: the semiconductor deviceaccording to claim 22; and at least one of an operation key and abattery.